diff --git a/src/backend/executor/nodeMergejoin.c b/src/backend/executor/nodeMergejoin.c
index 6988a6c6429..a049bc4ae05 100644
--- a/src/backend/executor/nodeMergejoin.c
+++ b/src/backend/executor/nodeMergejoin.c
@@ -893,11 +893,10 @@ ExecMergeJoin(PlanState *pstate)
 
 						if (compareResult == 0)
 							node->mj_JoinState = EXEC_MJ_JOINTUPLES;
-						else
-						{
-							Assert(compareResult < 0);
+						else if (compareResult < 0)
 							node->mj_JoinState = EXEC_MJ_NEXTOUTER;
-						}
+						else	/* compareResult > 0 should not happen */
+							elog(ERROR, "mergejoin input data is out of order");
 						break;
 					case MJEVAL_NONMATCHABLE:
 
@@ -1087,7 +1086,7 @@ ExecMergeJoin(PlanState *pstate)
 
 					node->mj_JoinState = EXEC_MJ_JOINTUPLES;
 				}
-				else
+				else if (compareResult > 0)
 				{
 					/* ----------------
 					 *	if the new outer tuple didn't match the marked inner
@@ -1106,7 +1105,6 @@ ExecMergeJoin(PlanState *pstate)
 					 *	no more inners, no more matches are possible.
 					 * ----------------
 					 */
-					Assert(compareResult > 0);
 					innerTupleSlot = node->mj_InnerTupleSlot;
 
 					/* reload comparison data for current inner */
@@ -1140,6 +1138,8 @@ ExecMergeJoin(PlanState *pstate)
 							return NULL;
 					}
 				}
+				else			/* compareResult < 0 should not happen */
+					elog(ERROR, "mergejoin input data is out of order");
 				break;
 
 				/*----------------------------------------------------------