fix error message for multiindex.fillna

Author: MarcoGorelli

doc/source/whatsnew/v3.0.0.rst

@@ -817,6 +817,7 @@ Other
 - Bug in :meth:`DataFrame.transform` that was returning the wrong order unless the index was monotonically increasing. (:issue:`57069`)
 - Bug in :meth:`DataFrame.where` where using a non-bool type array in the function would return a ``ValueError`` instead of a ``TypeError`` (:issue:`56330`)
 - Bug in :meth:`Index.sort_values` when passing a key function that turns values into tuples, e.g. ``key=natsort.natsort_key``, would raise ``TypeError`` (:issue:`56081`)
+- Bug in :meth:`MultiIndex.fillna` error message was referring to ``isna`` instead of ``fillna`` (:issue:`60974`)
 - Bug in :meth:`Series.diff` allowing non-integer values for the ``periods`` argument. (:issue:`56607`)
 - Bug in :meth:`Series.dt` methods in :class:`ArrowDtype` that were returning incorrect values. (:issue:`57355`)
 - Bug in :meth:`Series.isin` raising ``TypeError`` when series is large (>10**6) and ``values`` contains NA (:issue:`60678`)

pandas/core/indexes/multi.py

@@ -1760,7 +1760,7 @@ def fillna(self, value):
         """
         fillna is not implemented for MultiIndex
         """
-        raise NotImplementedError("isna is not defined for MultiIndex")
+        raise NotImplementedError("fillna is not defined for MultiIndex")
 
     @doc(Index.dropna)
     def dropna(self, how: AnyAll = "any") -> MultiIndex:

pandas/tests/indexing/multiindex/test_multiindex.py

@@ -249,3 +249,8 @@ def test_groupyby_rename_categories_operation_with_multiindex(self, operation):
         expected = getattr(a, operation)(b.sort_index(ascending=False))
 
         tm.assert_series_equal(result, expected)
+
+    def test_multiindex_fillna(self):
+        mi = MultiIndex.from_tuples([("a",)])
+        with pytest.raises(NotImplementedError, match="fillna"):
+            mi.fillna(0)