BUG: merge between partial index and index fails when result is empty

doc/source/whatsnew/v1.2.0.rst

@@ -357,6 +357,7 @@ Reshaping
 - Bug in :func:`union_indexes` where input index names are not preserved in some cases. Affects :func:`concat` and :class:`DataFrame` constructor (:issue:`13475`)
 - Bug in func :meth:`crosstab` when using multiple columns with ``margins=True`` and ``normalize=True`` (:issue:`35144`)
 - Bug in :meth:`DataFrame.agg` with ``func={'name':<FUNC>}`` incorrectly raising ``TypeError`` when ``DataFrame.columns==['Name']`` (:issue:`36212`)
+- Fixed bug in :func:`merge` where an error was raised when performing an inner join with partial index and ``right_index`` when no overlap between indices (:issue:`33814`)
 -
 
 Sparse

pandas/core/reshape/merge.py

@@ -834,7 +834,7 @@ def _maybe_add_join_keys(self, result, left_indexer, right_indexer):
                 # make sure to just use the right values
                 mask = left_indexer == -1
                 if mask.all():
-                    key_col = rvals
+                    key_col = Index(rvals)
                 else:
                     key_col = Index(lvals).where(~mask, rvals)
 

pandas/tests/reshape/merge/test_merge.py

@@ -2234,3 +2234,15 @@ def test_categorical_non_unique_monotonic(n_categories):
         index=left_index,
     )
     tm.assert_frame_equal(expected, result)
+
+
+def test_merge_empty_right_index_left_on():
+    # GH 33814
+    df1 = pd.DataFrame({"a": [1], "b": [2]}).set_index(["a", "b"])
+    df2 = pd.DataFrame({"b": [1]}).set_index(["b"])
+    expected = pd.DataFrame({"a": [], "b": []}, dtype=np.int64).set_index(["a", "b"])
+    result = pd.merge(df1, df2, left_on=["b"], right_index=True)
+    tm.assert_frame_equal(result, expected)
+
+    result = pd.merge(df2, df1, left_index=True, right_on=["b"])
+    tm.assert_frame_equal(result, expected)