diff --git a/src/backend/commands/indexcmds.c b/src/backend/commands/indexcmds.c
index 05afcb0088d..9d48032617c 100644
--- a/src/backend/commands/indexcmds.c
+++ b/src/backend/commands/indexcmds.c
@@ -773,6 +773,9 @@ DefineIndex(Oid relationId,
 	UnregisterSnapshot(snapshot);
 	InvalidateCatalogSnapshot();
 
+	/* Temporary debug check: we should be advertising we have no snapshots. */
+	Assert(MyPgXact->xmin == InvalidTransactionId);
+
 	/*
 	 * The index is now valid in the sense that it contains all currently
 	 * interesting tuples.  But since it might not contain tuples deleted just