diff --git a/django/contrib/admin/sites.py b/django/contrib/admin/sites.py
index c16ab6a110..cbf1666804 100644
--- a/django/contrib/admin/sites.py
+++ b/django/contrib/admin/sites.py
@@ -49,14 +49,15 @@ class AdminSite(object):
 
         If a model is already registered, this will raise AlreadyRegistered.
         """
+        if not admin_class:
+            admin_class = ModelAdmin
+
         # Don't import the humongous validation code unless required
         if admin_class and settings.DEBUG:
             from django.contrib.admin.validation import validate
         else:
             validate = lambda model, adminclass: None
 
-        if not admin_class:
-            admin_class = ModelAdmin
         if isinstance(model_or_iterable, ModelBase):
             model_or_iterable = [model_or_iterable]
         for model in model_or_iterable: