diff --git a/lib/discovery.c b/lib/discovery.c
index d38004d..a6077cb 100644
--- a/lib/discovery.c
+++ b/lib/discovery.c
@@ -127,8 +127,10 @@ iscsi_process_text_reply(struct iscsi_context *iscsi, struct iscsi_pdu *pdu,
 	if (in->hdr[1] != ISCSI_PDU_TEXT_FINAL) {
 		iscsi_set_error(iscsi, "unsupported flags in text "
 				"reply %02x", in->hdr[1]);
-		pdu->callback(iscsi, SCSI_STATUS_ERROR, NULL,
-			      pdu->private_data);
+		if (pdu->callback) {
+			pdu->callback(iscsi, SCSI_STATUS_ERROR, NULL,
+			              pdu->private_data);
+		}
 		return -1;
 	}
 
@@ -141,8 +143,10 @@ iscsi_process_text_reply(struct iscsi_context *iscsi, struct iscsi_pdu *pdu,
 			iscsi_set_error(iscsi, "NUL not found after offset %ld "
 					"when parsing discovery data",
 					(long)(ptr - in->data));
-			pdu->callback(iscsi, SCSI_STATUS_ERROR, NULL,
-				      pdu->private_data);
+			if (pdu->callback) {
+				pdu->callback(iscsi, SCSI_STATUS_ERROR, NULL,
+				              pdu->private_data);
+			}
 			iscsi_free_discovery_addresses(iscsi, targets);
 			return -1;
 		}
@@ -161,8 +165,10 @@ iscsi_process_text_reply(struct iscsi_context *iscsi, struct iscsi_pdu *pdu,
 				iscsi_set_error(iscsi, "Failed to allocate "
 						"data for new discovered "
 						"target");
-				pdu->callback(iscsi, SCSI_STATUS_ERROR, NULL,
-					      pdu->private_data);
+				if (pdu->callback) {
+					pdu->callback(iscsi, SCSI_STATUS_ERROR, NULL,
+					              pdu->private_data);
+				}
 				iscsi_free_discovery_addresses(iscsi, targets);
 				return -1;
 			}
@@ -171,8 +177,10 @@ iscsi_process_text_reply(struct iscsi_context *iscsi, struct iscsi_pdu *pdu,
 				iscsi_set_error(iscsi, "Failed to allocate "
 						"data for new discovered "
 						"target name");
-				pdu->callback(iscsi, SCSI_STATUS_ERROR, NULL,
-					      pdu->private_data);
+				if (pdu->callback) {
+					pdu->callback(iscsi, SCSI_STATUS_ERROR, NULL,
+					              pdu->private_data);
+				}
 				iscsi_free(iscsi, target);
 				target = NULL;
 				iscsi_free_discovery_addresses(iscsi, targets);
@@ -186,8 +194,10 @@ iscsi_process_text_reply(struct iscsi_context *iscsi, struct iscsi_pdu *pdu,
 			if (targets == NULL) {
 				iscsi_set_error(iscsi, "Invalid discovery "
 						"reply");
-				pdu->callback(iscsi, SCSI_STATUS_ERROR, NULL,
-					      pdu->private_data);
+				if (pdu->callback) {
+					pdu->callback(iscsi, SCSI_STATUS_ERROR, NULL,
+					              pdu->private_data);
+				}
 				iscsi_free_discovery_addresses(iscsi, targets);
 				return -1;
 			}
@@ -195,8 +205,10 @@ iscsi_process_text_reply(struct iscsi_context *iscsi, struct iscsi_pdu *pdu,
 			if (portal == NULL) {
 				iscsi_set_error(iscsi, "Failed to malloc "
 						"portal structure");
-				pdu->callback(iscsi, SCSI_STATUS_ERROR, NULL,
-					      pdu->private_data);
+				if (pdu->callback) {
+					pdu->callback(iscsi, SCSI_STATUS_ERROR, NULL,
+					              pdu->private_data);
+				}
 				iscsi_free_discovery_addresses(iscsi, targets);
 				return -1;
 			}
@@ -209,16 +221,20 @@ iscsi_process_text_reply(struct iscsi_context *iscsi, struct iscsi_pdu *pdu,
 				iscsi_set_error(iscsi, "Failed to allocate "
 						"data for new discovered "
 						"target address");
-				pdu->callback(iscsi, SCSI_STATUS_ERROR, NULL,
-					      pdu->private_data);
+				if (pdu->callback) {
+					pdu->callback(iscsi, SCSI_STATUS_ERROR, NULL,
+					              pdu->private_data);
+				}
 				iscsi_free_discovery_addresses(iscsi, targets);
 				return -1;
 			}
 		} else {
 			iscsi_set_error(iscsi, "Don't know how to handle "
 					"discovery string : %s", ptr);
-			pdu->callback(iscsi, SCSI_STATUS_ERROR, NULL,
-				      pdu->private_data);
+			if (pdu->callback) {
+				pdu->callback(iscsi, SCSI_STATUS_ERROR, NULL,
+				              pdu->private_data);
+			}
 			iscsi_free_discovery_addresses(iscsi, targets);
 			return -1;
 		}
@@ -227,7 +243,9 @@ iscsi_process_text_reply(struct iscsi_context *iscsi, struct iscsi_pdu *pdu,
 		size -= len + 1;
 	}
 
-	pdu->callback(iscsi, SCSI_STATUS_GOOD, targets, pdu->private_data);
+	if (pdu->callback) {
+		pdu->callback(iscsi, SCSI_STATUS_GOOD, targets, pdu->private_data);
+	}
 	iscsi_free_discovery_addresses(iscsi, targets);
 
 	return 0;
diff --git a/lib/iscsi-command.c b/lib/iscsi-command.c
index c879ecf..a2c4463 100644
--- a/lib/iscsi-command.c
+++ b/lib/iscsi-command.c
@@ -59,15 +59,19 @@ iscsi_scsi_response_cb(struct iscsi_context *iscsi, int status,
 	case SCSI_STATUS_CANCELLED:
 	case SCSI_STATUS_TIMEOUT:
 		scsi_cbdata->task->status = status;
-		scsi_cbdata->callback(iscsi, status, scsi_cbdata->task,
-				      scsi_cbdata->private_data);
+		if (scsi_cbdata->callback) {
+			scsi_cbdata->callback(iscsi, status, scsi_cbdata->task,
+			                      scsi_cbdata->private_data);
+		}
 		return;
 	default:
 		scsi_cbdata->task->status = SCSI_STATUS_ERROR;
 		iscsi_set_error(iscsi, "Cant handle  scsi status %d yet.",
-				status);
-		scsi_cbdata->callback(iscsi, SCSI_STATUS_ERROR, scsi_cbdata->task,
-				      scsi_cbdata->private_data);
+		                status);
+		if (scsi_cbdata->callback) {
+			scsi_cbdata->callback(iscsi, SCSI_STATUS_ERROR, scsi_cbdata->task,
+			                      scsi_cbdata->private_data);
+		}
 	}
 }
 
@@ -90,13 +94,7 @@ iscsi_send_data_out(struct iscsi_context *iscsi, struct iscsi_pdu *cmd_pdu,
 		if (pdu == NULL) {
 			iscsi_set_error(iscsi, "Out-of-memory, Failed to allocate "
 				"scsi data out pdu.");
-			ISCSI_LIST_REMOVE(&iscsi->outqueue, cmd_pdu);
-			ISCSI_LIST_REMOVE(&iscsi->waitpdu, cmd_pdu);
-			cmd_pdu->callback(iscsi, SCSI_STATUS_ERROR, NULL,
-				     cmd_pdu->private_data);
-			iscsi_free_pdu(iscsi, cmd_pdu);
-			return -1;
-
+			goto error;
 		}
 		pdu->scsi_cbdata.task         = cmd_pdu->scsi_cbdata.task;
 		/* set the cmdsn in the pdu struct so we can compare with
@@ -134,19 +132,23 @@ iscsi_send_data_out(struct iscsi_context *iscsi, struct iscsi_pdu *cmd_pdu,
 		if (iscsi_queue_pdu(iscsi, pdu) != 0) {
 			iscsi_set_error(iscsi, "Out-of-memory: failed to queue iscsi "
 				"scsi pdu.");
-			ISCSI_LIST_REMOVE(&iscsi->outqueue, cmd_pdu);
-			ISCSI_LIST_REMOVE(&iscsi->waitpdu, cmd_pdu);
-			cmd_pdu->callback(iscsi, SCSI_STATUS_ERROR, NULL,
-				     cmd_pdu->private_data);
-			iscsi_free_pdu(iscsi, cmd_pdu);
-			iscsi_free_pdu(iscsi, pdu);
-			return -1;
+			goto error;
 		}
 
 		tot_len -= len;
 		offset  += len;
 	}
 	return 0;
+
+error:
+	ISCSI_LIST_REMOVE(&iscsi->outqueue, cmd_pdu);
+	ISCSI_LIST_REMOVE(&iscsi->waitpdu, cmd_pdu);
+	if (cmd_pdu->callback) {
+		cmd_pdu->callback(iscsi, SCSI_STATUS_ERROR, NULL,
+						  cmd_pdu->private_data);
+	}
+	iscsi_free_pdu(iscsi, cmd_pdu);
+	return -1;
 }
 
 static int
@@ -370,15 +372,19 @@ iscsi_process_scsi_reply(struct iscsi_context *iscsi, struct iscsi_pdu *pdu,
 	if ((flags&ISCSI_PDU_DATA_FINAL) == 0) {
 		iscsi_set_error(iscsi, "scsi response pdu but Final bit is "
 				"not set: 0x%02x.", flags);
-		pdu->callback(iscsi, SCSI_STATUS_ERROR, task,
-			      pdu->private_data);
+		if (pdu->callback) {
+			pdu->callback(iscsi, SCSI_STATUS_ERROR, task,
+			              pdu->private_data);
+		}
 		return -1;
 	}
 	if ((flags&ISCSI_PDU_DATA_ACK_REQUESTED) != 0) {
 		iscsi_set_error(iscsi, "scsi response asked for ACK "
 				"0x%02x.", flags);
-		pdu->callback(iscsi, SCSI_STATUS_ERROR, task,
-			      pdu->private_data);
+		if (pdu->callback) {
+			pdu->callback(iscsi, SCSI_STATUS_ERROR, task,
+			              pdu->private_data);
+		}
 		return -1;
 	}
 
@@ -392,8 +398,10 @@ iscsi_process_scsi_reply(struct iscsi_context *iscsi, struct iscsi_pdu *pdu,
 		if (response != 0) {
 			iscsi_set_error(iscsi, "protocol error: flags %#02x;"
 					" response %#02x.", flags, response);
-			pdu->callback(iscsi, SCSI_STATUS_ERROR, task,
-				      pdu->private_data);
+			if (pdu->callback) {
+				pdu->callback(iscsi, SCSI_STATUS_ERROR, task,
+				              pdu->private_data);
+			}
 			return -1;
 		}
 		task->residual = scsi_get_uint32(&in->hdr[44]);
@@ -421,8 +429,10 @@ iscsi_process_scsi_reply(struct iscsi_context *iscsi, struct iscsi_pdu *pdu,
 		pdu->indata.data = NULL;
 		pdu->indata.size = 0;
 
-		pdu->callback(iscsi, SCSI_STATUS_GOOD, task,
-			      pdu->private_data);
+		if (pdu->callback) {
+			pdu->callback(iscsi, SCSI_STATUS_GOOD, task,
+			              pdu->private_data);
+		}
 		break;
 	case SCSI_STATUS_CHECK_CONDITION:
 		task->datain.size = in->data_pos;
@@ -440,39 +450,52 @@ iscsi_process_scsi_reply(struct iscsi_context *iscsi, struct iscsi_pdu *pdu,
 				task->sense.key,
 				scsi_sense_ascq_str(task->sense.ascq),
 				task->sense.ascq);
-		pdu->callback(iscsi, SCSI_STATUS_CHECK_CONDITION, task,
-			      pdu->private_data);
+		if (pdu->callback) {
+			pdu->callback(iscsi, SCSI_STATUS_CHECK_CONDITION, task,
+			              pdu->private_data);
+		}
 		break;
 	case SCSI_STATUS_RESERVATION_CONFLICT:
 		iscsi_set_error(iscsi, "RESERVATION CONFLICT");
-		pdu->callback(iscsi, SCSI_STATUS_RESERVATION_CONFLICT,
-			task, pdu->private_data);
+		if (pdu->callback) {
+			pdu->callback(iscsi, SCSI_STATUS_RESERVATION_CONFLICT,
+			              task, pdu->private_data);
+		}
 		break;
 	case SCSI_STATUS_TASK_SET_FULL:
 		iscsi_set_error(iscsi, "TASK_SET_FULL");
-		pdu->callback(iscsi, SCSI_STATUS_TASK_SET_FULL,
-			task, pdu->private_data);
+		if (pdu->callback) {
+			pdu->callback(iscsi, SCSI_STATUS_TASK_SET_FULL,
+			              task, pdu->private_data);
+		}
 		break;
 	case SCSI_STATUS_ACA_ACTIVE:
 		iscsi_set_error(iscsi, "ACA_ACTIVE");
-		pdu->callback(iscsi, SCSI_STATUS_ACA_ACTIVE,
-			task, pdu->private_data);
+		if (pdu->callback) {
+			pdu->callback(iscsi, SCSI_STATUS_ACA_ACTIVE,
+			              task, pdu->private_data);
+		}
 		break;
 	case SCSI_STATUS_TASK_ABORTED:
 		iscsi_set_error(iscsi, "TASK_ABORTED");
-		pdu->callback(iscsi, SCSI_STATUS_TASK_ABORTED,
-			task, pdu->private_data);
+		if (pdu->callback) {
+			pdu->callback(iscsi, SCSI_STATUS_TASK_ABORTED,
+			              task, pdu->private_data);
+		}
 		break;
 	case SCSI_STATUS_BUSY:
 		iscsi_set_error(iscsi, "BUSY");
-		pdu->callback(iscsi, SCSI_STATUS_BUSY,
-			task, pdu->private_data);
+		if (pdu->callback) {
+			pdu->callback(iscsi, SCSI_STATUS_BUSY,
+			              task, pdu->private_data);
+		}
 		break;
 	default:
 		iscsi_set_error(iscsi, "Unknown SCSI status :%d.", status);
-
-		pdu->callback(iscsi, SCSI_STATUS_ERROR, task,
-			      pdu->private_data);
+		if (pdu->callback) {
+			pdu->callback(iscsi, SCSI_STATUS_ERROR,
+			              task, pdu->private_data);
+		}
 		return -1;
 	}
 
@@ -492,8 +515,10 @@ iscsi_process_scsi_data_in(struct iscsi_context *iscsi, struct iscsi_pdu *pdu,
 	if ((flags&ISCSI_PDU_DATA_ACK_REQUESTED) != 0) {
 		iscsi_set_error(iscsi, "scsi response asked for ACK "
 				"0x%02x.", flags);
-		pdu->callback(iscsi, SCSI_STATUS_ERROR, task,
-			      pdu->private_data);
+		if (pdu->callback) {
+			pdu->callback(iscsi, SCSI_STATUS_ERROR, task,
+			              pdu->private_data);
+		}
 		return -1;
 	}
 	dsl = scsi_get_uint32(&in->hdr[4]) & 0x00ffffff;
@@ -550,7 +575,9 @@ iscsi_process_scsi_data_in(struct iscsi_context *iscsi, struct iscsi_pdu *pdu,
 	pdu->indata.data = NULL;
 	pdu->indata.size = 0;
 
-	pdu->callback(iscsi, status, task, pdu->private_data);
+	if (pdu->callback) {
+		pdu->callback(iscsi, status, task, pdu->private_data);
+	}
 
 	return 0;
 }
diff --git a/lib/login.c b/lib/login.c
index bd91953..bc1c50e 100644
--- a/lib/login.c
+++ b/lib/login.c
@@ -1082,8 +1082,10 @@ iscsi_process_login_reply(struct iscsi_context *iscsi, struct iscsi_pdu *pdu,
 			iscsi_set_error(iscsi, "NUL not found after offset %ld "
 					"when parsing login data",
 					(long)((unsigned char *)ptr - in->data));
-			pdu->callback(iscsi, SCSI_STATUS_ERROR, NULL,
-				      pdu->private_data);
+			if (pdu->callback) {
+				pdu->callback(iscsi, SCSI_STATUS_ERROR, NULL,
+				              pdu->private_data);
+			}
 			return -1;
 		}
 
@@ -1161,8 +1163,10 @@ iscsi_process_login_reply(struct iscsi_context *iscsi, struct iscsi_pdu *pdu,
 			if (len-9 > MAX_CHAP_C_LENGTH) {
 				iscsi_set_error(iscsi, "Wrong length of CHAP_C received from"
 						" target (%d, max: %d)", len-9, MAX_CHAP_C_LENGTH);
-				pdu->callback(iscsi, SCSI_STATUS_ERROR, NULL,
-						  pdu->private_data);
+				if (pdu->callback) {
+					pdu->callback(iscsi, SCSI_STATUS_ERROR, NULL,
+					              pdu->private_data);
+				}
 				return 0;
 			}
 			*iscsi->chap_c = '\0';
@@ -1175,8 +1179,10 @@ iscsi_process_login_reply(struct iscsi_context *iscsi, struct iscsi_pdu *pdu,
 				iscsi_set_error(iscsi, "Failed to log in to"
 						" target. Wrong CHAP targetname"
 						" received: %s", ptr + 7);
-				pdu->callback(iscsi, SCSI_STATUS_ERROR, NULL,
-					      pdu->private_data);
+				if (pdu->callback) {
+					pdu->callback(iscsi, SCSI_STATUS_ERROR, NULL,
+					              pdu->private_data);
+				}
 				return 0;
 			}
 			must_have_chap_n = 0;
@@ -1188,8 +1194,10 @@ iscsi_process_login_reply(struct iscsi_context *iscsi, struct iscsi_pdu *pdu,
 			if (len != 9 + 2 * CHAP_R_SIZE) {
 				iscsi_set_error(iscsi, "Wrong size of CHAP_R"
 						" received from target.");
-				pdu->callback(iscsi, SCSI_STATUS_ERROR, NULL,
-					      pdu->private_data);
+				if (pdu->callback) {
+					pdu->callback(iscsi, SCSI_STATUS_ERROR, NULL,
+					              pdu->private_data);
+				}
 				return 0;
 			}
 			for (i = 0; i < CHAP_R_SIZE; i++) {
@@ -1199,8 +1207,10 @@ iscsi_process_login_reply(struct iscsi_context *iscsi, struct iscsi_pdu *pdu,
 					iscsi_set_error(iscsi, "Authentication "
 						"failed. Invalid CHAP_R "
 						"response from the target");
-					pdu->callback(iscsi, SCSI_STATUS_ERROR,
-						      NULL, pdu->private_data);
+					if (pdu->callback) {
+						pdu->callback(iscsi, SCSI_STATUS_ERROR,
+						              NULL, pdu->private_data);
+					}
 					return 0;
 				}
 			}
@@ -1213,32 +1223,40 @@ iscsi_process_login_reply(struct iscsi_context *iscsi, struct iscsi_pdu *pdu,
 
 	if (status == SCSI_STATUS_REDIRECT && iscsi->target_address[0]) {
 		ISCSI_LOG(iscsi, 2, "target requests redirect to %s",iscsi->target_address);
-		pdu->callback(iscsi, SCSI_STATUS_REDIRECT, NULL,
-				  pdu->private_data);
+		if (pdu->callback) {
+			pdu->callback(iscsi, SCSI_STATUS_REDIRECT, NULL,
+			              pdu->private_data);
+		}
 		return 0;
 	}
 
 	if (status != 0) {
 		iscsi_set_error(iscsi, "Failed to log in to target. Status: %s(%d)",
 				       login_error_str(status), status);
-		pdu->callback(iscsi, SCSI_STATUS_ERROR, NULL,
-			      pdu->private_data);
+		if (pdu->callback) {
+			pdu->callback(iscsi, SCSI_STATUS_ERROR, NULL,
+			              pdu->private_data);
+		}
 		return 0;
 	}
 
 	if (must_have_chap_n) {
 		iscsi_set_error(iscsi, "Failed to log in to target. "
 				"It did not return CHAP_N during SECNEG.");
-		pdu->callback(iscsi, SCSI_STATUS_ERROR, NULL,
-			      pdu->private_data);
+		if (pdu->callback) {
+			pdu->callback(iscsi, SCSI_STATUS_ERROR, NULL,
+			              pdu->private_data);
+		}
 		return 0;
 	}
 
 	if (must_have_chap_r) {
 		iscsi_set_error(iscsi, "Failed to log in to target. "
 				"It did not return CHAP_R during SECNEG.");
-		pdu->callback(iscsi, SCSI_STATUS_ERROR, NULL,
-			      pdu->private_data);
+		if (pdu->callback) {
+			pdu->callback(iscsi, SCSI_STATUS_ERROR, NULL,
+			              pdu->private_data);
+		}
 		return 0;
 	}
 
@@ -1256,7 +1274,9 @@ iscsi_process_login_reply(struct iscsi_context *iscsi, struct iscsi_pdu *pdu,
 	} else {
 		if (iscsi_login_async(iscsi, pdu->callback, pdu->private_data) != 0) {
 			iscsi_set_error(iscsi, "Failed to send continuation login pdu");
-			pdu->callback(iscsi, SCSI_STATUS_ERROR, NULL, pdu->private_data);
+			if (pdu->callback) {
+				pdu->callback(iscsi, SCSI_STATUS_ERROR, NULL, pdu->private_data);
+			}
 		}
 	}
 
@@ -1315,7 +1335,9 @@ struct iscsi_in_pdu *in _U_)
 {
 	iscsi->is_loggedin = 0;
 	ISCSI_LOG(iscsi, 2, "logout successful");
-	pdu->callback(iscsi, SCSI_STATUS_GOOD, NULL, pdu->private_data);
+	if (pdu->callback) {
+		pdu->callback(iscsi, SCSI_STATUS_GOOD, NULL, pdu->private_data);
+	}
 
 	return 0;
 }
diff --git a/lib/pdu.c b/lib/pdu.c
index ebffe17..dc04244 100644
--- a/lib/pdu.c
+++ b/lib/pdu.c
@@ -365,7 +365,7 @@ int iscsi_process_reject(struct iscsi_context *iscsi,
 
 	if (pdu->callback) {
 		pdu->callback(iscsi, SCSI_STATUS_ERROR, NULL,
-					pdu->private_data);
+		              pdu->private_data);
 	}
 
 	ISCSI_LIST_REMOVE(&iscsi->waitpdu, pdu);
@@ -708,7 +708,7 @@ iscsi_timeout_scan(struct iscsi_context *iscsi)
 		iscsi_dump_pdu_header(iscsi, pdu->outdata.data);
 		if (pdu->callback) {
 			pdu->callback(iscsi, SCSI_STATUS_TIMEOUT,
-					  NULL, pdu->private_data);
+			              NULL, pdu->private_data);
 		}
 		iscsi_free_pdu(iscsi, pdu);
 	}
@@ -728,7 +728,7 @@ iscsi_timeout_scan(struct iscsi_context *iscsi)
 		iscsi_dump_pdu_header(iscsi, pdu->outdata.data);
 		if (pdu->callback) {
 			pdu->callback(iscsi, SCSI_STATUS_TIMEOUT,
-					  NULL, pdu->private_data);
+			              NULL, pdu->private_data);
 		}
 		iscsi_free_pdu(iscsi, pdu);
 	}
diff --git a/lib/task_mgmt.c b/lib/task_mgmt.c
index b00b6fa..1fd3f5f 100644
--- a/lib/task_mgmt.c
+++ b/lib/task_mgmt.c
@@ -78,7 +78,6 @@ iscsi_task_mgmt_async(struct iscsi_context *iscsi,
 	/* rcmdsn */
 	iscsi_pdu_set_rcmdsn(pdu, rcmdsn);
 
-	
 	pdu->callback     = cb;
 	pdu->private_data = private_data;
 
@@ -97,7 +96,9 @@ iscsi_process_task_mgmt_reply(struct iscsi_context *iscsi, struct iscsi_pdu *pdu
 {
 	uint32_t response = in->hdr[2];
 
-	pdu->callback(iscsi, SCSI_STATUS_GOOD, &response, pdu->private_data);
+	if (pdu->callback) {
+		pdu->callback(iscsi, SCSI_STATUS_GOOD, &response, pdu->private_data);
+	}
 	return 0;
 }