diff --git a/python-package/xgboost/core.py b/python-package/xgboost/core.py
index ba76f31a6..4565dc5d0 100644
--- a/python-package/xgboost/core.py
+++ b/python-package/xgboost/core.py
@@ -643,7 +643,7 @@ class Booster(object):
     def __copy__(self):
         return self.__deepcopy__()
 
-    def __deepcopy__(self):
+    def __deepcopy__(self, memo):
         return Booster(model_file=self.save_raw())
 
     def copy(self):