diff --git a/doc/src/sgml/bki.sgml b/doc/src/sgml/bki.sgml
index e3ba73a9a8..d53cbf810f 100644
--- a/doc/src/sgml/bki.sgml
+++ b/doc/src/sgml/bki.sgml
@@ -122,10 +122,7 @@
    if they are fixed-width and are not preceded by any nullable column.
    Where this rule is inadequate, you can force correct marking by using
    <literal>BKI_FORCE_NOT_NULL</literal>
-   and <literal>BKI_FORCE_NULL</literal> annotations as needed.  But note
-   that <literal>NOT NULL</literal> constraints are only enforced in the
-   executor, not against tuples that are generated by random C code,
-   so care is still needed when manually creating or updating catalog rows.
+   and <literal>BKI_FORCE_NULL</literal> annotations as needed.
   </para>
 
   <para>
diff --git a/src/backend/catalog/indexing.c b/src/backend/catalog/indexing.c
index a5f5bc46a9..674e6a8321 100644
--- a/src/backend/catalog/indexing.c
+++ b/src/backend/catalog/indexing.c
@@ -18,6 +18,8 @@
 #include "access/htup_details.h"
 #include "catalog/index.h"
 #include "catalog/indexing.h"
+#include "catalog/pg_subscription.h"
+#include "catalog/pg_subscription_rel.h"
 #include "executor/executor.h"
 #include "utils/rel.h"
 
@@ -164,6 +166,53 @@ CatalogIndexInsert(CatalogIndexState indstate, HeapTuple heapTuple)
 	ExecDropSingleTupleTableSlot(slot);
 }
 
+/*
+ * Subroutine to verify that catalog constraints are honored.
+ *
+ * Tuples inserted via CatalogTupleInsert/CatalogTupleUpdate are generally
+ * "hand made", so that it's possible that they fail to satisfy constraints
+ * that would be checked if they were being inserted by the executor.  That's
+ * a coding error, so we only bother to check for it in assert-enabled builds.
+ */
+#ifdef USE_ASSERT_CHECKING
+
+static void
+CatalogTupleCheckConstraints(Relation heapRel, HeapTuple tup)
+{
+	/*
+	 * Currently, the only constraints implemented for system catalogs are
+	 * attnotnull constraints.
+	 */
+	if (HeapTupleHasNulls(tup))
+	{
+		TupleDesc	tupdesc = RelationGetDescr(heapRel);
+		bits8	   *bp = tup->t_data->t_bits;
+
+		for (int attnum = 0; attnum < tupdesc->natts; attnum++)
+		{
+			Form_pg_attribute thisatt = TupleDescAttr(tupdesc, attnum);
+
+			/*
+			 * Through an embarrassing oversight, pre-v13 installations have
+			 * pg_subscription.subslotname and pg_subscription_rel.srsublsn
+			 * marked as attnotnull, which they should not be.  Ignore those
+			 * flags.
+			 */
+			Assert(!(thisatt->attnotnull && att_isnull(attnum, bp) &&
+					 !((thisatt->attrelid == SubscriptionRelationId &&
+						thisatt->attnum == Anum_pg_subscription_subslotname) ||
+					   (thisatt->attrelid == SubscriptionRelRelationId &&
+						thisatt->attnum == Anum_pg_subscription_rel_srsublsn))));
+		}
+	}
+}
+
+#else							/* !USE_ASSERT_CHECKING */
+
+#define CatalogTupleCheckConstraints(heapRel, tup)  ((void) 0)
+
+#endif							/* USE_ASSERT_CHECKING */
+
 /*
  * CatalogTupleInsert - do heap and indexing work for a new catalog tuple
  *
@@ -182,6 +231,8 @@ CatalogTupleInsert(Relation heapRel, HeapTuple tup)
 	CatalogIndexState indstate;
 	Oid			oid;
 
+	CatalogTupleCheckConstraints(heapRel, tup);
+
 	indstate = CatalogOpenIndexes(heapRel);
 
 	oid = simple_heap_insert(heapRel, tup);
@@ -206,6 +257,8 @@ CatalogTupleInsertWithInfo(Relation heapRel, HeapTuple tup,
 {
 	Oid			oid;
 
+	CatalogTupleCheckConstraints(heapRel, tup);
+
 	oid = simple_heap_insert(heapRel, tup);
 
 	CatalogIndexInsert(indstate, tup);
@@ -229,6 +282,8 @@ CatalogTupleUpdate(Relation heapRel, ItemPointer otid, HeapTuple tup)
 {
 	CatalogIndexState indstate;
 
+	CatalogTupleCheckConstraints(heapRel, tup);
+
 	indstate = CatalogOpenIndexes(heapRel);
 
 	simple_heap_update(heapRel, otid, tup);
@@ -249,6 +304,8 @@ void
 CatalogTupleUpdateWithInfo(Relation heapRel, ItemPointer otid, HeapTuple tup,
 						   CatalogIndexState indstate)
 {
+	CatalogTupleCheckConstraints(heapRel, tup);
+
 	simple_heap_update(heapRel, otid, tup);
 
 	CatalogIndexInsert(indstate, tup);