BUG-24212 fix when other_index has incompatible dtype

pandas/core/reshape/merge.py

@@ -809,10 +809,18 @@ def _create_join_index(self, index, other_index, indexer,
                 # if values missing (-1) from target index,
                 # take from other_index instead
                 join_list = join_index.to_numpy()
-                other_list = other_index.take(other_indexer).to_numpy()
-                join_list[mask] = other_list[mask]
-                join_index = Index(join_list, dtype=join_index.dtype,
-                                   name=join_index.name)
+                try:
+                    other_list = other_index.take(other_indexer).to_numpy()
+                    join_list[mask] = other_list[mask]
+                    join_index = Index(join_list, dtype=other_index.dtype,
+                                       name=other_index.name)
+                except ValueError:
+                    # if other_index does not have a compatible dtype, naively
+                    # replace missing values by their column position in other
+                    naive_index = np.arange(len(other_index))
+                    other_list = naive_index.take(other_indexer)
+                    join_list[mask] = other_list[mask]
+                    join_index = Index(join_list, name=other_index.name)
         return join_index
 
     def _get_merge_keys(self):

pandas/tests/reshape/merge/test_merge.py

@@ -973,6 +973,21 @@ def test_merge_right_index_right(self):
                             how='right')
         tm.assert_frame_equal(result, expected)
 
+    def test_merge_take_missing_values_from_index_of_other_dtype(self):
+        left = pd.DataFrame({'a': [1, 2, 3],
+                             'key': pd.Categorical(['a', 'a', 'b'],
+                                                   categories=list('abc'))})
+        right = pd.DataFrame({'b': [1, 2, 3]},
+                             index=pd.CategoricalIndex(['a', 'b', 'c']))
+        result = left.merge(right, left_on='key',
+                            right_index=True, how='right')
+        expected = pd.DataFrame({'a': [1, 2, 3, None],
+                                 'key': pd.Categorical(['a', 'a', 'b', 'c']),
+                                 'b': [1, 1, 2, 3]},
+                                index=[0, 1, 2, 2])
+        expected = expected.reindex(columns=['a', 'key', 'b'])
+        tm.assert_frame_equal(result, expected)
+
 
 def _check_merge(x, y):
     for how in ['inner', 'left', 'outer']: