diff --git a/tests/oauth2/test_id_token.py b/tests/oauth2/test_id_token.py
index f3da663..7f89547 100644
--- a/tests/oauth2/test_id_token.py
+++ b/tests/oauth2/test_id_token.py
@@ -18,6 +18,7 @@
 import pytest
 
 from google.auth import exceptions
+import google.auth.transport
 from google.oauth2 import id_token
 
 
@@ -28,7 +29,7 @@
     if data is not None:
         response.data = json.dumps(data).encode('utf-8')
 
-    return mock.Mock(return_value=response)
+    return mock.Mock(return_value=response, spec=google.auth.transport.Request)
 
 
 def test__fetch_certs_success():
@@ -37,7 +38,7 @@
 
     returned_certs = id_token._fetch_certs(request, mock.sentinel.cert_url)
 
-    request.assert_called_once_with('GET', mock.sentinel.cert_url)
+    request.assert_called_once_with(mock.sentinel.cert_url, method='GET')
     assert returned_certs == certs
 
 
@@ -47,7 +48,7 @@
     with pytest.raises(exceptions.TransportError):
         id_token._fetch_certs(request, mock.sentinel.cert_url)
 
-    request.assert_called_once_with('GET', mock.sentinel.cert_url)
+    request.assert_called_once_with(mock.sentinel.cert_url, method='GET')
 
 
 @mock.patch('google.auth.jwt.decode', autospec=True)
diff --git a/tests/test__helpers.py b/tests/test__helpers.py
index d413adc..b067faa 100644
--- a/tests/test__helpers.py
+++ b/tests/test__helpers.py
@@ -151,3 +151,19 @@
 
     for case, expected in cases:
         assert _helpers.string_to_scopes(case) == expected
+
+
+def test_padded_urlsafe_b64decode():
+    cases = [
+        ('YQ==', b'a'),
+        ('YQ', b'a'),
+        ('YWE=', b'aa'),
+        ('YWE', b'aa'),
+        ('YWFhYQ==', b'aaaa'),
+        ('YWFhYQ', b'aaaa'),
+        ('YWFhYWE=', b'aaaaa'),
+        ('YWFhYWE', b'aaaaa'),
+    ]
+
+    for case, expected in cases:
+        assert _helpers.padded_urlsafe_b64decode(case) == expected