diff --git a/google/auth/_oauth2client.py b/google/auth/_oauth2client.py
index 71fd7bf..afe7dc4 100644
--- a/google/auth/_oauth2client.py
+++ b/google/auth/_oauth2client.py
@@ -25,6 +25,7 @@
 
 from google.auth import _helpers
 import google.auth.app_engine
+import google.auth.compute_engine
 import google.oauth2.credentials
 import google.oauth2.service_account
 
@@ -37,7 +38,7 @@
         ImportError('oauth2client is not installed.'), caught_exc)
 
 try:
-    import oauth2client.contrib.appengine
+    import oauth2client.contrib.appengine  # pytype: disable=import-error
     _HAS_APPENGINE = True
 except ImportError:
     _HAS_APPENGINE = False
diff --git a/google/auth/app_engine.py b/google/auth/app_engine.py
index f47dae1..91ba842 100644
--- a/google/auth/app_engine.py
+++ b/google/auth/app_engine.py
@@ -28,10 +28,12 @@
 from google.auth import credentials
 from google.auth import crypt
 
+# pytype: disable=import-error
 try:
     from google.appengine.api import app_identity
 except ImportError:
     app_identity = None
+# pytype: enable=import-error
 
 
 class Signer(crypt.Signer):
diff --git a/google/auth/jwt.py b/google/auth/jwt.py
index ef23db2..3805f37 100644
--- a/google/auth/jwt.py
+++ b/google/auth/jwt.py
@@ -738,7 +738,7 @@
         parts = urllib.parse.urlsplit(url)
         # Strip query string and fragment
         audience = urllib.parse.urlunsplit(
-            (parts.scheme, parts.netloc, parts.path, None, None))
+            (parts.scheme, parts.netloc, parts.path, "", ""))
         token = self._get_jwt_for_audience(audience)
         self.apply(headers, token=token)