diff --git a/pandas/core/groupby/groupby.py b/pandas/core/groupby/groupby.py
index d1f51eeae1f69..67b5c483fcb97 100644
--- a/pandas/core/groupby/groupby.py
+++ b/pandas/core/groupby/groupby.py
@@ -5844,6 +5844,7 @@ def _wrap_idxmax_idxmin(self, res: NDFrameT) -> NDFrameT:
             if isinstance(index, MultiIndex):
                 index = index.to_flat_index()
             values = res._values
+            assert isinstance(values, np.ndarray)
             na_value = na_value_for_dtype(index.dtype, compat=False)
             if isinstance(res, Series):
                 # mypy: expression has type "Series", variable has type "NDFrameT"