diff --git a/superset/models/core.py b/superset/models/core.py
index b8293b59d34f5..7392e8796a571 100644
--- a/superset/models/core.py
+++ b/superset/models/core.py
@@ -598,7 +598,12 @@ def get_sqla_engine(self, schema=None, nullpool=False, user_name=None):
             params['poolclass'] = NullPool
         uri = self.db_engine_spec.adjust_database_uri(uri, schema)
         if self.impersonate_user:
-            uri.username = user_name if user_name else g.user.username
+            eff_username = uri.username
+            if user_name:
+                eff_username = user_name
+            elif hasattr(g, 'user') and g.user.username:
+                eff_username = g.user.username
+            uri.username = eff_username
         return create_engine(uri, **params)
 
     def get_reserved_words(self):