diff --git a/src/backend/executor/execPartition.c b/src/backend/executor/execPartition.c
index 84ccd7d457d..0374476ffad 100644
--- a/src/backend/executor/execPartition.c
+++ b/src/backend/executor/execPartition.c
@@ -876,7 +876,7 @@ ExecInitPartitionInfo(ModifyTableState *mtstate, EState *estate,
 	 * reference and make copy for this relation, converting stuff that
 	 * references attribute numbers to match this relation's.
 	 *
-	 * This duplicates much of the logic in ExecInitMerge(), so something
+	 * This duplicates much of the logic in ExecInitMerge(), so if something
 	 * changes there, look here too.
 	 */
 	if (node && node->operation == CMD_MERGE)
@@ -956,6 +956,8 @@ ExecInitPartitionInfo(ModifyTableState *mtstate, EState *estate,
 												  NULL);
 					break;
 				case CMD_DELETE:
+				case CMD_NOTHING:
+					/* Nothing to do */
 					break;
 
 				default:
diff --git a/src/backend/executor/nodeModifyTable.c b/src/backend/executor/nodeModifyTable.c
index 87c820276a8..309e27f8b5f 100644
--- a/src/backend/executor/nodeModifyTable.c
+++ b/src/backend/executor/nodeModifyTable.c
@@ -3784,7 +3784,7 @@ ExecInitMerge(ModifyTableState *mtstate, EState *estate)
 				case CMD_NOTHING:
 					break;
 				default:
-					elog(ERROR, "unknown operation");
+					elog(ERROR, "unknown action in MERGE WHEN clause");
 					break;
 			}
 		}
diff --git a/src/test/regress/expected/merge.out b/src/test/regress/expected/merge.out
index c8ecc8b6b4a..bcd29668297 100644
--- a/src/test/regress/expected/merge.out
+++ b/src/test/regress/expected/merge.out
@@ -2060,6 +2060,23 @@ SELECT * FROM pa_target ORDER BY tid;
   15 |    1500 | initial
 (8 rows)
 
+ROLLBACK;
+-- bug #18871: ExecInitPartitionInfo()'s handling of DO NOTHING actions
+BEGIN;
+TRUNCATE pa_target;
+MERGE INTO pa_target t
+  USING (VALUES (10, 100)) AS s(sid, delta)
+  ON t.tid = s.sid
+  WHEN NOT MATCHED THEN
+    INSERT VALUES (1, 10, 'inserted by merge')
+  WHEN MATCHED THEN
+    DO NOTHING;
+SELECT * FROM pa_target ORDER BY tid, val;
+ tid | balance |        val        
+-----+---------+-------------------
+   1 |      10 | inserted by merge
+(1 row)
+
 ROLLBACK;
 DROP TABLE pa_target CASCADE;
 -- The target table is partitioned in the same way, but this time by attaching
diff --git a/src/test/regress/sql/merge.sql b/src/test/regress/sql/merge.sql
index 07b6295b3ba..f7a19c0e7dd 100644
--- a/src/test/regress/sql/merge.sql
+++ b/src/test/regress/sql/merge.sql
@@ -1271,6 +1271,19 @@ MERGE INTO pa_target t
 SELECT * FROM pa_target ORDER BY tid;
 ROLLBACK;
 
+-- bug #18871: ExecInitPartitionInfo()'s handling of DO NOTHING actions
+BEGIN;
+TRUNCATE pa_target;
+MERGE INTO pa_target t
+  USING (VALUES (10, 100)) AS s(sid, delta)
+  ON t.tid = s.sid
+  WHEN NOT MATCHED THEN
+    INSERT VALUES (1, 10, 'inserted by merge')
+  WHEN MATCHED THEN
+    DO NOTHING;
+SELECT * FROM pa_target ORDER BY tid, val;
+ROLLBACK;
+
 DROP TABLE pa_target CASCADE;
 
 -- The target table is partitioned in the same way, but this time by attaching