diff --git a/src/backend/commands/indexcmds.c b/src/backend/commands/indexcmds.c
index f2dcc1c51f..dda0dcb8aa 100644
--- a/src/backend/commands/indexcmds.c
+++ b/src/backend/commands/indexcmds.c
@@ -1197,14 +1197,25 @@ DefineIndex(Oid relationId,
 	 * doing CREATE INDEX CONCURRENTLY, which would see our snapshot as one
 	 * they must wait for.  But first, save the snapshot's xmin to use as
 	 * limitXmin for GetCurrentVirtualXIDs().
-	 *
-	 * Our catalog snapshot could have the same effect, so drop that one too.
 	 */
 	limitXmin = snapshot->xmin;
 
 	PopActiveSnapshot();
 	UnregisterSnapshot(snapshot);
-	InvalidateCatalogSnapshot();
+
+	/*
+	 * The snapshot subsystem could still contain registered snapshots that
+	 * are holding back our process's advertised xmin; in particular, if
+	 * default_transaction_isolation = serializable, there is a transaction
+	 * snapshot that is still active.  The CatalogSnapshot is likewise a
+	 * hazard.  To ensure no deadlocks, we must commit and start yet another
+	 * transaction, and do our wait before any snapshot has been taken in it.
+	 */
+	CommitTransactionCommand();
+	StartTransactionCommand();
+
+	/* We should now definitely not be advertising any xmin. */
+	Assert(MyPgXact->xmin == InvalidTransactionId);
 
 	/*
 	 * The index is now valid in the sense that it contains all currently