diff --git a/api/tools/prepare_sql_tiles.py b/api/tools/prepare_sql_tiles.py
index 599a7f5..4f0fd47 100755
--- a/api/tools/prepare_sql_tiles.py
+++ b/api/tools/prepare_sql_tiles.py
@@ -48,7 +48,7 @@ DROP FUNCTION IF EXISTS {fname}(integer, integer, integer{extra_args_types});
 CREATE FUNCTION {fname}(zoom integer, x integer, y integer{extra_args_definitions})
 RETURNS {'TABLE(mvt bytea, key text)' if self.key_column else 'bytea'} AS $$
 {self.generate_sql()};
-$$ LANGUAGE SQL STABLE RETURNS NULL ON NULL INPUT;"""
+$$ LANGUAGE SQL STABLE CALLED ON NULL INPUT;"""
 
 EXTRA_ARGS = [
     # name, type, default