BUG: Put the checking a single-level selection from MultiIndex higher

pandas/core/groupby.py

@@ -2586,6 +2586,15 @@ 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:
@@ -2626,14 +2635,6 @@ def _get_grouper(obj, key=None, axis=0, level=None, sort=True,
     elif isinstance(key, BaseGrouper):
         return key, [], obj
 
-    if key is None and isinstance(group_axis, MultiIndex):
-        if is_list_like(level) and len(level) == 1:
-            level = level[0]
-
-        if level is not None and is_scalar(level):
-            key = group_axis.get_level_values(level)
-            level = None
-
     if not isinstance(key, (tuple, list)):
         keys = [key]
         match_axis_length = False