Add a string

Author: Daniel Saxton

pandas/tests/arrays/categorical/test_constructors.py

@@ -460,7 +460,7 @@ def test_constructor_with_categorical_categories(self):
 
     def test_construction_with_null(self, nulls_fixture):
         # https://github.com/pandas-dev/pandas/issues/31927
-        values = ["a", nulls_fixture]
+        values = ["a", nulls_fixture, "b"]
         result = Categorical(np.array(values, dtype=object))
         expected = Categorical(values)