diff --git a/imagekit/specs/__init__.py b/imagekit/specs/__init__.py
index dc33593..3878bc9 100644
--- a/imagekit/specs/__init__.py
+++ b/imagekit/specs/__init__.py
@@ -233,7 +233,7 @@ class SpecHost(object):
                 raise TypeError('You can provide either an image spec or'
                     ' arguments for the ImageSpec constructor, but not both.')
             else:
-                spec = type('Spec', (ImageSpec,), spec_args)  # TODO: Base class name on spec id?
+                spec = type('Spec', (ImageSpec,), dict((k, v) for k, v in spec_args.items() if v is not None))  # TODO: Base class name on spec id?
 
         self._original_spec = spec