diff --git a/src/googleapiclient/__init__.py b/src/googleapiclient/__init__.py
index 97977bb7..184219af 100644
--- a/src/googleapiclient/__init__.py
+++ b/src/googleapiclient/__init__.py
@@ -12,7 +12,7 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-__version__ = "1.6.3"
+__version__ = "1.6.4"
 
 # Set default logging handler to avoid "No handler found" warnings.
 import logging
diff --git a/src/googleapiclient/_auth.py b/src/googleapiclient/_auth.py
index 92f2796f..ada4ffb1 100644
--- a/src/googleapiclient/_auth.py
+++ b/src/googleapiclient/_auth.py
@@ -19,11 +19,15 @@ import httplib2
 try:
     import google.auth
     import google.auth.credentials
-    import google_auth_httplib2
     HAS_GOOGLE_AUTH = True
 except ImportError:  # pragma: NO COVER
     HAS_GOOGLE_AUTH = False
 
+try:
+    import google_auth_httplib2
+except ImportError:  # pragma: NO COVER
+    google_auth_httplib2 = None
+
 try:
     import oauth2client
     import oauth2client.client
@@ -88,6 +92,12 @@ def authorized_http(credentials):
 
     if HAS_GOOGLE_AUTH and isinstance(
             credentials, google.auth.credentials.Credentials):
+        if google_auth_httplib2 is None:
+            raise ValueError(
+                'Credentials from google.auth specified, but '
+                'google-api-python-client is unable to use these credentials '
+                'unless google-auth-httplib2 is installed. Please install '
+                'google-auth-httplib2.')
         return google_auth_httplib2.AuthorizedHttp(credentials,
                                                    http=build_http())
     else: