diff --git a/python/pyspark/ml/wrapper.py b/python/pyspark/ml/wrapper.py
index 7b0893e2cdadc..253705bde913e 100644
--- a/python/pyspark/ml/wrapper.py
+++ b/python/pyspark/ml/wrapper.py
@@ -166,7 +166,7 @@ def __init__(self, java_model):
         self._java_obj = java_model
         self.uid = java_model.uid()
 
-    def copy(self, extra={}):
+    def copy(self, extra=None):
         """
         Creates a copy of this instance with the same uid and some
         extra params. This implementation first calls Params.copy and
@@ -175,6 +175,8 @@ def copy(self, extra={}):
         :param extra: Extra parameters to copy to the new instance
         :return: Copy of this instance
         """
+        if extra is None:
+            extra = dict()
         that = super(JavaModel, self).copy(extra)
         that._java_obj = self._java_obj.copy(self._empty_java_param_map())
         that._transfer_params_to_java()