BUG: Change the location of the checking a single-level selection from MultiIndex

Author: Licht-T

pandas/core/groupby.py

@@ -2586,19 +2586,18 @@ def _get_grouper(obj, key=None, axis=0, level=None, sort=True,
     """
     group_axis = obj._get_axis(axis)
 
-    if key is None and level is not None and \
-       isinstance(group_axis, MultiIndex):
-        if is_list_like(level) and len(level) == 1:
-            level = level[0]
-
-        if is_scalar(level):
-            key = group_axis.get_level_values(level)
-            level = None
-
     # validate that the passed level is compatible with the passed
     # axis of the object
     if level is not None:
-        if not isinstance(group_axis, MultiIndex):
+        if isinstance(group_axis, MultiIndex):
+            if is_list_like(level) and len(level) == 1:
+                level = level[0]
+
+            if key is None and is_scalar(level):
+                key = group_axis.get_level_values(level)
+                level = None
+
+        else:
             # allow level to be a length-one list-like object
             # (e.g., level=[0])
             # GH 13901