diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c
index 16c86ffcc0f..34f8bc801af 100644
--- a/src/backend/commands/tablecmds.c
+++ b/src/backend/commands/tablecmds.c
@@ -18688,8 +18688,14 @@ ATExecAttachPartition(List **wqueue, Relation rel, PartitionCmd *cmd,
 	 * constraint as well.
 	 */
 	partBoundConstraint = get_qual_from_partbound(rel, cmd->bound);
-	partConstraint = list_concat(partBoundConstraint,
-								 RelationGetPartitionQual(rel));
+
+	/*
+	 * Use list_concat_copy() to avoid modifying partBoundConstraint in place,
+	 * since it’s needed later to construct the constraint expression for
+	 * validating against the default partition, if any.
+	 */
+	partConstraint = list_concat_copy(partBoundConstraint,
+									  RelationGetPartitionQual(rel));
 
 	/* Skip validation if there are no constraints to validate. */
 	if (partConstraint)