diff --git a/imagekit/admin.py b/imagekit/admin.py
index cc24d29..94d9cf5 100644
--- a/imagekit/admin.py
+++ b/imagekit/admin.py
@@ -24,8 +24,8 @@ class AdminThumbnail(object):
         thumbnail = getattr(obj, self.image_field, None)
 
         if not thumbnail:
-            raise Exception('The property {0} is not defined on {1}.'.format(
-                obj, self.image_field))
+            raise Exception('The property %s is not defined on %s.' % \
+                    (self.image_field, obj.__class__.__name__))
 
         original_image = getattr(thumbnail, 'source_file', None) or thumbnail
         template = self.template or 'imagekit/admin/thumbnail.html'