diff --git a/src/backend/parser/parse_utilcmd.c b/src/backend/parser/parse_utilcmd.c
index ee2d2b54a1..973eab6ae2 100644
--- a/src/backend/parser/parse_utilcmd.c
+++ b/src/backend/parser/parse_utilcmd.c
@@ -3698,8 +3698,16 @@ transformPartitionCmd(CreateStmtContext *cxt, PartitionCmd *cmd)
 														 cmd->bound);
 			break;
 		case RELKIND_PARTITIONED_INDEX:
-			/* nothing to check */
-			Assert(cmd->bound == NULL);
+
+			/*
+			 * A partitioned index cannot have a partition bound set.  ALTER
+			 * INDEX prevents that with its grammar, but not ALTER TABLE.
+			 */
+			if (cmd->bound != NULL)
+				ereport(ERROR,
+						(errcode(ERRCODE_INVALID_OBJECT_DEFINITION),
+						 errmsg("\"%s\" is not a partitioned table",
+								RelationGetRelationName(parentRel))));
 			break;
 		case RELKIND_RELATION:
 			/* the table must be partitioned */
diff --git a/src/test/regress/expected/indexing.out b/src/test/regress/expected/indexing.out
index ec1d4eaef4..f78865ef81 100644
--- a/src/test/regress/expected/indexing.out
+++ b/src/test/regress/expected/indexing.out
@@ -121,6 +121,23 @@ Partition of: idxparti2
 No partition constraint
 btree, for table "public.idxpart1"
 
+-- Forbid ALTER TABLE when attaching or detaching an index to a partition.
+create index idxpart_c on only idxpart (c);
+create index idxpart1_c on idxpart1 (c);
+alter table idxpart_c attach partition idxpart1_c for values from (10) to (20);
+ERROR:  "idxpart_c" is not a partitioned table
+alter index idxpart_c attach partition idxpart1_c;
+select relname, relpartbound from pg_class
+  where relname in ('idxpart_c', 'idxpart1_c')
+  order by relname;
+  relname   | relpartbound 
+------------+--------------
+ idxpart1_c | 
+ idxpart_c  | 
+(2 rows)
+
+alter table idxpart_c detach partition idxpart1_c;
+ERROR:  "idxpart_c" is not a table
 drop table idxpart;
 -- If a partition already has an index, don't create a duplicative one
 create table idxpart (a int, b int) partition by range (a, b);
diff --git a/src/test/regress/sql/indexing.sql b/src/test/regress/sql/indexing.sql
index f6a3767918..35d159f41b 100644
--- a/src/test/regress/sql/indexing.sql
+++ b/src/test/regress/sql/indexing.sql
@@ -63,6 +63,16 @@ alter table idxpart attach partition idxpart1 for values from (0) to (10);
 \d idxpart1
 \d+ idxpart1_a_idx
 \d+ idxpart1_b_c_idx
+
+-- Forbid ALTER TABLE when attaching or detaching an index to a partition.
+create index idxpart_c on only idxpart (c);
+create index idxpart1_c on idxpart1 (c);
+alter table idxpart_c attach partition idxpart1_c for values from (10) to (20);
+alter index idxpart_c attach partition idxpart1_c;
+select relname, relpartbound from pg_class
+  where relname in ('idxpart_c', 'idxpart1_c')
+  order by relname;
+alter table idxpart_c detach partition idxpart1_c;
 drop table idxpart;
 
 -- If a partition already has an index, don't create a duplicative one