diff --git a/entsoe/entsoe.py b/entsoe/entsoe.py
index 5355689..60b2743 100644
--- a/entsoe/entsoe.py
+++ b/entsoe/entsoe.py
@@ -1911,7 +1911,7 @@ def query_generation_per_plant(
         df = df.tz_convert(area.tz)
         # Truncation will fail if data is not sorted along the index in rare
         # cases. Ensure the dataframe is sorted:
-        df = df.sort_index(0)
+        df = df.sort_index(axis=0)
         df = df.truncate(before=start, after=end)
         return df