diff --git a/pandas/core/frame.py b/pandas/core/frame.py
index 6c3523f830e97..04ebaf26fc11c 100644
--- a/pandas/core/frame.py
+++ b/pandas/core/frame.py
@@ -8548,7 +8548,7 @@ def idxmax(self, axis=0, skipna=True) -> Series:
         result = [index[i] if i >= 0 else np.nan for i in indices]
         return Series(result, index=self._get_agg_axis(axis))
 
-    def _get_agg_axis(self, axis_num):
+    def _get_agg_axis(self, axis_num: int) -> Index:
         """
         Let's be explicit about this.
         """