diff --git a/elephas/ml_model.py b/elephas/ml_model.py
index 5e4f951..90b5438 100644
--- a/elephas/ml_model.py
+++ b/elephas/ml_model.py
@@ -1,6 +1,7 @@
 from __future__ import absolute_import, print_function
 
 import numpy as np
+import copy
 import h5py
 import json
 
@@ -150,7 +151,7 @@ def _transform(self, df):
         """
         output_col = self.getOutputCol()
         label_col = self.getLabelCol()
-        new_schema = df.schema
+        new_schema = copy.deepcopy(df.schema)
         new_schema.add(StructField(output_col, StringType(), True))
 
         rdd = df.rdd.coalesce(1)
@@ -176,4 +177,4 @@ def load_ml_transformer(file_name):
     f = h5py.File(file_name, mode='r')
     elephas_conf = json.loads(f.attrs.get('distributed_config'))
     config = elephas_conf.get('config')
-    return ElephasTransformer(**config)
\ No newline at end of file
+    return ElephasTransformer(**config)