diff --git a/rest_framework/generics.py b/rest_framework/generics.py
index 6969f1be3..c80f5b202 100644
--- a/rest_framework/generics.py
+++ b/rest_framework/generics.py
@@ -84,29 +84,31 @@ class GenericAPIView(views.APIView):
         keyword arguments in the url conf.
         """
         queryset = self.filter_queryset(self.get_queryset())
-
-        # Perform the lookup filtering.
-        lookup_url_kwarg = self.lookup_url_kwarg or self.lookup_field
-
-        assert lookup_url_kwarg in self.kwargs, (
-                'Expected view %s to be called with a URL keyword argument '
-                'named "%s". Fix your URL conf, or set the `.lookup_field` '
-                'attribute on the view correctly.' %
-                (self.__class__.__name__, lookup_url_kwarg)
-        )
-
         lookup_arg = self.lookup_arg
 
-        assert lookup_arg in self.request.GET, (
-                'Expected view %s to be called with a URL keyword argument '
-                'named "%s". Fix your URL conf, or set the `.lookup_arg` '
-                'attribute on the view correctly.' %
-                (self.__class__.__name__, lookup_arg)
-        )
+        if lookup_arg is None:
 
-        filter_kwargs = {self.lookup_field: self.request.GET[lookup_arg]} \
-            if lookup_arg is not None \
-            else {self.lookup_field: self.kwargs[lookup_url_kwarg]}
+            # Perform the lookup filtering.
+            lookup_url_kwarg = self.lookup_url_kwarg or self.lookup_field
+
+            assert lookup_url_kwarg in self.kwargs, (
+                    'Expected view %s to be called with a URL keyword argument '
+                    'named "%s". Fix your URL conf, or set the `.lookup_field` '
+                    'attribute on the view correctly.' %
+                    (self.__class__.__name__, lookup_url_kwarg)
+            )
+            filter_kwargs = {self.lookup_field: self.kwargs[lookup_url_kwarg]}
+
+        else:
+
+            assert lookup_arg in self.request.GET, (
+                    'Expected view %s to be called with a URL keyword argument '
+                    'named "%s". Fix your URL conf, or set the `.lookup_arg` '
+                    'attribute on the view correctly.' %
+                    (self.__class__.__name__, lookup_arg)
+            )
+
+            filter_kwargs = {self.lookup_field: self.request.GET[lookup_arg]}
 
         obj = get_object_or_404(queryset, **filter_kwargs)