diff --git a/gpiozero/pins/mock.py b/gpiozero/pins/mock.py
index c260c6f..91d4b6a 100644
--- a/gpiozero/pins/mock.py
+++ b/gpiozero/pins/mock.py
@@ -49,7 +49,7 @@ class MockPin(Pin):
             self.clear_states()
             return self
         if old_pin.__class__ != cls:
-            raise ValueError('pin %d is already in use as a %s' % (number, old_pin.__class__))
+            raise ValueError('pin %d is already in use as a %s' % (number, old_pin.__class__.__name__))
         return old_pin
 
     def __repr__(self):
diff --git a/tests/test_mock_pin.py b/tests/test_mock_pin.py
index 857e340..92e1d3d 100644
--- a/tests/test_mock_pin.py
+++ b/tests/test_mock_pin.py
@@ -22,6 +22,8 @@ def teardown_function(function):
 # enough to get reasonable coverage but it's by no means comprehensive...
 
 def test_mock_pin_init():
+    with pytest.raises(TypeError):
+        MockPin()
     with pytest.raises(ValueError):
         MockPin(60)
     assert MockPin(2).number == 2
@@ -53,6 +55,8 @@ def test_mock_pin_init_twice_different_pin():
     assert pin2.number == pin1.number+1
 
 def test_mock_pwm_pin_init():
+    with pytest.raises(TypeError):
+        MockPWMPin()
     with pytest.raises(ValueError):
         MockPWMPin(60)
     assert MockPWMPin(2).number == 2