diff --git a/pydal/objects.py b/pydal/objects.py
index f0d896a86..661782c28 100644
--- a/pydal/objects.py
+++ b/pydal/objects.py
@@ -1441,7 +1441,7 @@ def with_alias(self, alias):
     # GIS expressions
 
     def st_asgeojson(self, precision=15, options=0, version=1):
-        return Expression(self.db, self.db._adapter.ST_ASGEOJSON, self,
+        return Expression(self.db, self._dialect.st_asgeojson, self,
                           dict(precision=precision, options=options,
                                version=version), 'string')