diff --git a/imagekit/models/fields/__init__.py b/imagekit/models/fields/__init__.py
index 1042d83..9556991 100644
--- a/imagekit/models/fields/__init__.py
+++ b/imagekit/models/fields/__init__.py
@@ -7,7 +7,9 @@ from ...registry import register
 
 
 class SpecHostField(SpecHost):
-    def set_spec_id(self, cls, name):
+    def _set_spec_id(self, cls, name):
+        spec_id = getattr(self, 'spec_id', None)
+
         # Generate a spec_id to register the spec with. The default spec id is
         # "<app>:<model>_<field>"
         if not getattr(self, 'spec_id', None):
@@ -59,7 +61,7 @@ class ImageSpecField(SpecHostField):
             source = image_fields[0]
 
         setattr(cls, name, ImageSpecFileDescriptor(self, name, source))
-        self.set_spec_id(cls, name)
+        self._set_spec_id(cls, name)
 
         # Add the model and field as a source for this spec id
         register.source_group(self.spec_id, ImageFieldSourceGroup(cls, source))
@@ -92,7 +94,7 @@ class ProcessedImageField(models.ImageField, SpecHostField):
                 height_field, **kwargs)
 
     def contribute_to_class(self, cls, name):
-        self.set_spec_id(cls, name)
+        self._set_spec_id(cls, name)
         return super(ProcessedImageField, self).contribute_to_class(cls, name)