--- a/testing/marionette/harness/marionette_harness/tests/harness_unit/conftest.py
+++ b/testing/marionette/harness/marionette_harness/tests/harness_unit/conftest.py
@@ -87,14 +87,14 @@ def mock_httpd(request):
     """ Mock httpd instance """
     httpd = MagicMock(spec=FixtureServer)
     return httpd
 
 
 @pytest.fixture
 def mock_marionette(request):
     """ Mock marionette instance """
-    marionette_class = MagicMock(spec=Marionette)
+    marionette = MagicMock(spec=Marionette())
     if 'has_crashed' in request.funcargnames:
-        marionette_class.check_for_crash.return_value = request.getfuncargvalue(
+        marionette.check_for_crash.return_value = request.getfuncargvalue(
             'has_crashed'
         )
-    return marionette_class
+    return marionette

--- a/testing/marionette/harness/marionette_harness/tests/harness_unit/test_marionette_runner.py
+++ b/testing/marionette/harness/marionette_harness/tests/harness_unit/test_marionette_runner.py
@@ -20,17 +20,17 @@ def runner(mach_parsed_kwargs):
 
 @pytest.fixture
 def mock_runner(runner, mock_marionette, monkeypatch):
     """
     MarionetteTestRunner instance with mocked-out
     self.marionette and other properties,
     to enable testing runner.run_tests().
     """
-    runner.driverclass = mock_marionette
+    runner.driverclass = Mock(return_value=mock_marionette)
     for attr in ['run_test_set', '_capabilities']:
         setattr(runner, attr, Mock())
     runner._appName = 'fake_app'
     # simulate that browser runs with e10s by default
     runner._appinfo = {'browserTabsRemoteAutostart': True}
     monkeypatch.setattr('marionette_harness.runner.base.mozversion', Mock())
     return runner