diff --git a/tests/system-test/2-query/union.py b/tests/system-test/2-query/union.py
index 5b3e27affa..97647b8549 100644
--- a/tests/system-test/2-query/union.py
+++ b/tests/system-test/2-query/union.py
@@ -201,7 +201,7 @@ class TDTestCase:
                         )
                     )
 
-        return sqls
+        return filter(None, sqls)
 
     def __get_type(self, col):
         if tdSql.cursor.istype(col, "BOOL"):