diff --git a/gluon/sqlhtml.py b/gluon/sqlhtml.py
index b37e6b93..b8cc55e5 100644
--- a/gluon/sqlhtml.py
+++ b/gluon/sqlhtml.py
@@ -29,7 +29,7 @@ from gluon.html import URL, FIELDSET, P, DEFAULT_PASSWORD_DISPLAY
 from pydal.base import DEFAULT
 from pydal.objects import Table, Row, Expression, Field, Set, Rows
 from pydal.adapters.base import CALLABLETYPES
-from pydal.helpers.methods import smart_query, bar_encode,  _repr_ref
+from pydal.helpers.methods import smart_query, bar_encode, _repr_ref, merge_tablemaps
 from pydal.helpers.classes import Reference, SQLCustomType
 from gluon.storage import Storage
 from gluon.utils import md5_hash
@@ -2329,7 +2329,7 @@ class SQLFORM(FORM):
             if not isinstance(left, (list, tuple)):
                 left = [left]
             for join in left:
-                tablenames += db._adapter.tables(join)
+                tablenames = merge_tablemaps(tablenames, db._adapter.tables(join))
         tables = [db[tablename] for tablename in tablenames]
         if fields:
             # add missing tablename to virtual fields