diff --git a/baselines/common/segment_tree.py b/baselines/common/segment_tree.py
index cb386ecdb5..b825ebdb86 100644
--- a/baselines/common/segment_tree.py
+++ b/baselines/common/segment_tree.py
@@ -112,7 +112,7 @@ def find_prefixsum_idx(self, prefixsum):
 
         Parameters
         ----------
-        perfixsum: float
+        prefixsum: float
             upperbound on the sum of array prefix
 
         Returns