diff --git a/polymorphic/polymorphic_model.py b/polymorphic/polymorphic_model.py
index 76b4d51..daf2689 100644
--- a/polymorphic/polymorphic_model.py
+++ b/polymorphic/polymorphic_model.py
@@ -110,7 +110,7 @@ class PolymorphicModel(six.with_metaclass(PolymorphicModelBase, models.Model)):
 
         # Protect against bad imports (dumpdata without --natural) or other
         # issues missing with the ContentType models.
-        if not issubclass(model, self.__class__):
+        if model is not None and not issubclass(model, self.__class__):
             raise RuntimeError("ContentType {0} for {1} #{2} does not point to a subclass!".format(
                 self.polymorphic_ctype_id, model, self.pk,
             ))