diff --git a/django/db/models/options.py b/django/db/models/options.py
index 826d773faa..b5d8b1bb9a 100644
--- a/django/db/models/options.py
+++ b/django/db/models/options.py
@@ -194,8 +194,10 @@ class Options(object):
             field.serialize = False
 
     def pk_index(self):
-        return [pos for pos, field in enumerate(self.fields)
-                if field == self.pk][0]
+        """
+        Returns the index of the primary key field in the self.fields list.
+        """
+        return self.fields.index(self.pk)
 
     def setup_proxy(self, target):
         """
@@ -537,9 +539,3 @@ class Options(object):
             #        objects.append(opts)
             self._ordered_objects = objects
         return self._ordered_objects
-
-    def pk_index(self):
-        """
-        Returns the index of the primary key field in the self.fields list.
-        """
-        return self.fields.index(self.pk)