Fix types

Author: dcherian

xarray/core/groupby.py

@@ -1049,7 +1049,7 @@ def quantile(
            The American Statistician, 50(4), pp. 361-365, 1996
         """
         if dim is None:
-            dim = self._group_dim
+            dim = (self._group_dim,)
 
         # Dataset.quantile does this, do it for flox to ensure same output.
         q = np.asarray(q, dtype=np.float64)

xarray/groupers.py

@@ -106,6 +106,7 @@ def __init__(
             self.unique_coord = unique_coord
 
         if coords is None:
+            assert not isinstance(self.unique_coord, _DummyGroup)
             self.coords = _coordinates_from_variable(self.unique_coord)
         else:
             self.coords = coords