Index: django/db/models/options.py
===================================================================
--- django/db/models/options.py (revision 6263)
+++ django/db/models/options.py (working copy)
@@ -186,6 +186,8 @@
             rel_objs = []
             for klass in get_models():
                 for f in klass._meta.many_to_many:
+                    if f.rel and isinstance(f.rel.to, str):
+                        raise Exception("class %s has unresolved string relation" % klass)
                     if f.rel and self == f.rel.to._meta:
                         rel_objs.append(RelatedObject(f.rel.to, klass, f))
             if app_cache_ready():