diff --git a/gcloud/client.py b/gcloud/client.py
index 5fa0716660d3..ac14e85de705 100644
--- a/gcloud/client.py
+++ b/gcloud/client.py
@@ -132,21 +132,26 @@ class _ClientProjectMixin(object):
                     passed falls back to the default inferred from the
                     environment.
 
-    :raises: :class:`ValueError` if the project is neither passed in nor
-             set in the environment.
+    :raises: :class:`EnvironmentError` if the project is neither passed in nor
+             set in the environment. :class:`ValueError` if the project value
+             is invalid.
     """
 
     def __init__(self, project=None):
-        project = _determine_default_project(project)
+        project = self._determine_default(project)
         if project is None:
-            raise ValueError('Project was not passed and could not be '
-                             'determined from the environment.')
+            raise EnvironmentError('Project was not passed and could not be '
+                                   'determined from the environment.')
         if isinstance(project, six.binary_type):
             project = project.decode('utf-8')
         if not isinstance(project, six.string_types):
             raise ValueError('Project must be a string.')
         self.project = project
 
+    @staticmethod
+    def _determine_default(project):
+        return _determine_default_project(project)
+
 
 class JSONClient(Client, _ClientProjectMixin):
     """Client to for Google JSON-based API.
diff --git a/gcloud/datastore/client.py b/gcloud/datastore/client.py
index 22900d6d557e..e4e066966445 100644
--- a/gcloud/datastore/client.py
+++ b/gcloud/datastore/client.py
@@ -18,6 +18,7 @@
 from gcloud._helpers import _LocalStack
 from gcloud._helpers import _app_engine_id
 from gcloud._helpers import _compute_engine_id
+from gcloud.client import _ClientProjectMixin
 from gcloud.client import Client as _BaseClient
 from gcloud.datastore import helpers
 from gcloud.datastore.connection import Connection
@@ -155,7 +156,7 @@ def _extended_lookup(connection, project, key_pbs,
     return results
 
 
-class Client(_BaseClient):
+class Client(_BaseClient, _ClientProjectMixin):
     """Convenience wrapper for invoking APIs/factories w/ a project.
 
     :type project: string
@@ -180,14 +181,15 @@ class Client(_BaseClient):
 
     def __init__(self, project=None, namespace=None,
                  credentials=None, http=None):
-        project = _determine_default_project(project)
-        if project is None:
-            raise EnvironmentError('Project could not be inferred.')
-        self.project = project
+        _ClientProjectMixin.__init__(self, project=project)
         self.namespace = namespace
         self._batch_stack = _LocalStack()
         super(Client, self).__init__(credentials, http)
 
+    @staticmethod
+    def _determine_default(project):
+        return _determine_default_project(project)
+
     def _push_batch(self, batch):
         """Push a batch/transaction onto our stack.
 
diff --git a/gcloud/test_client.py b/gcloud/test_client.py
index 18ae03ad3323..caed2c862731 100644
--- a/gcloud/test_client.py
+++ b/gcloud/test_client.py
@@ -171,7 +171,7 @@ def mock_determine_proj(project):
             return None
 
         with _Monkey(client, _determine_default_project=mock_determine_proj):
-            self.assertRaises(ValueError, self._makeOne)
+            self.assertRaises(EnvironmentError, self._makeOne)
 
         self.assertEqual(FUNC_CALLS, [(None, '_determine_default_project')])