BUG: Categorical.fillna with non-category tuple (pandas-dev#41914)

Author: jbrockmendel

doc/source/whatsnew/v1.3.0.rst

@@ -875,6 +875,7 @@ Categorical
 - Bug in constructing a :class:`DataFrame` from an ``ndarray`` and a :class:`CategoricalDtype` (:issue:`38857`)
 - Bug in setting categorical values into an object-dtype column in a :class:`DataFrame` (:issue:`39136`)
 - Bug in :meth:`DataFrame.reindex` was raising an ``IndexError`` when the new index contained duplicates and the old index was a :class:`CategoricalIndex` (:issue:`38906`)
+- Bug in :meth:`Categorical.fillna` with a tuple-like category raising ``NotImplementedError`` instead of ``ValueError`` when filling with a non-category tuple (:issue:`41914`)
 
 Datetimelike
 ^^^^^^^^^^^^

pandas/core/arrays/categorical.py

@@ -2030,7 +2030,8 @@ def _validate_setitem_value(self, value):
 
         from pandas import Index
 
-        to_add = Index(rvalue).difference(self.categories)
+        # tupleize_cols=False for e.g. test_fillna_iterable_category GH#41914
+        to_add = Index(rvalue, tupleize_cols=False).difference(self.categories)
 
         # no assignments of values not in categories, but it's always ok to set
         # something to np.nan

pandas/tests/arrays/categorical/test_missing.py

@@ -100,6 +100,13 @@ def test_fillna_iterable_category(self, named):
 
         tm.assert_categorical_equal(result, expected)
 
+        # Case where the Point is not among our categories; we want ValueError,
+        #  not NotImplementedError GH#41914
+        cat = Categorical(np.array([Point(1, 0), Point(0, 1), None], dtype=object))
+        msg = "Cannot setitem on a Categorical with a new category"
+        with pytest.raises(ValueError, match=msg):
+            cat.fillna(Point(0, 0))
+
     def test_fillna_array(self):
         # accept Categorical or ndarray value if it holds appropriate values
         cat = Categorical(["A", "B", "C", None, None])