diff --git a/pecan/core.py b/pecan/core.py
index b6c09ff..32185e5 100644
--- a/pecan/core.py
+++ b/pecan/core.py
@@ -368,6 +368,7 @@ class Pecan(object):
         # lookup the controller, respecting content-type as requested
         # by the file extension on the URI
         path = request.pecan['routing_path']
+        request.pecan['extension'] = None
 
         # attempt to guess the content type based on the file extension
         if not request.pecan['content_type'] and '.' in path.split('/')[-1]:
diff --git a/pecan/tests/test_base.py b/pecan/tests/test_base.py
index ba495b8..043df38 100644
--- a/pecan/tests/test_base.py
+++ b/pecan/tests/test_base.py
@@ -863,7 +863,7 @@ class TestFileTypeExtensions(unittest.TestCase):
         class RootController(object):
             @expose(content_type=None)
             def _default(self, *args):
-                ext = request.pecan.get('extension')
+                ext = request.pecan['extension']
                 assert len(args) == 1
                 if ext:
                     assert ext not in args[0]
@@ -916,7 +916,7 @@ class TestFileTypeExtensions(unittest.TestCase):
             @expose(content_type=None)
             def _default(self, *args):
                 assert 'example:x.tiny' in args
-                assert 'extension' not in request.pecan
+                assert request.pecan['extension'] is None
                 return 'SOME VALUE'
 
         app = TestApp(Pecan(RootController()))