diff --git a/xarray/core/groupby.py b/xarray/core/groupby.py
index 3c26c2129ae..9212549d4ad 100644
--- a/xarray/core/groupby.py
+++ b/xarray/core/groupby.py
@@ -996,7 +996,7 @@ def _combine(self, applied):
         if coord is not None and dim not in applied_example.dims:
             index, index_vars = create_default_index_implicit(coord)
             indexes = {k: index for k in index_vars}
-            combined = combined._overwrite_indexes(indexes, variables=index_vars)
+            combined = combined._overwrite_indexes(indexes, index_vars)
         combined = self._maybe_restore_empty_groups(combined)
         combined = self._maybe_unstack(combined)
         return combined
diff --git a/xarray/tests/test_groupby.py b/xarray/tests/test_groupby.py
index f866b68dfa8..327c8d34b18 100644
--- a/xarray/tests/test_groupby.py
+++ b/xarray/tests/test_groupby.py
@@ -934,6 +934,14 @@ def test_groupby_dataset_assign():
     assert_identical(actual, expected)
 
 
+def test_groupby_dataset_map_dataarray_func():
+    # regression GH6379
+    ds = xr.Dataset({"foo": ("x", [1, 2, 3, 4])}, coords={"x": [0, 0, 1, 1]})
+    actual = ds.groupby("x").map(lambda grp: grp.foo.mean())
+    expected = xr.DataArray([1.5, 3.5], coords={"x": [0, 1]}, dims="x", name="foo")
+    assert_identical(actual, expected)
+
+
 class TestDataArrayGroupBy:
     @pytest.fixture(autouse=True)
     def setup(self):