fixup! ENH: Parametrized CategoricalDtype

Author: TomAugspurger

pandas/tests/dtypes/test_dtypes.py

@@ -86,6 +86,11 @@ def test_construction_from_string(self):
         pytest.raises(
             TypeError, lambda: CategoricalDtype.construct_from_string('foo'))
 
+    def test_constructor_invalid(self):
+        with tm.assert_raises_regex(TypeError,
+                                    "CategoricalIndex.* must be called"):
+            CategoricalDtype("category")
+
     def test_is_dtype(self):
         assert CategoricalDtype.is_dtype(self.dtype)
         assert CategoricalDtype.is_dtype('category')