Index: django/contrib/gis/db/backend/postgis/field.py
===================================================================
--- django/contrib/gis/db/backend/postgis/field.py	(revision 1357)
+++ django/contrib/gis/db/backend/postgis/field.py	(working copy)
@@ -89,7 +89,7 @@
         SRID of the field.  Specifically, this routine will substitute in the
         ST_Transform() function call.
         """
-        if value is None or value.srid == self._srid:
+        if value is None or self._srid == -1 or value.srid == self._srid:
             return '%s'
         else:
             # Adding Transform() to the SQL placeholder.