diff --git a/imagekit/specs/__init__.py b/imagekit/specs/__init__.py
index c12acfa..3c65836 100644
--- a/imagekit/specs/__init__.py
+++ b/imagekit/specs/__init__.py
@@ -202,7 +202,9 @@ class SpecHost(object):
 
         """
         self.spec_id = id
-        register.generator(id, self._original_spec)
+
+        if self._original_spec:
+            register.generator(id, self._original_spec)
 
     def get_spec(self, source):
         """