diff --git a/python-client/wmill/wmill/s3_reader.py b/python-client/wmill/wmill/s3_reader.py
index e966e25f48..fb880b710c 100644
--- a/python-client/wmill/wmill/s3_reader.py
+++ b/python-client/wmill/wmill/s3_reader.py
@@ -30,14 +30,14 @@ class S3BufferedReader(BufferedReader):
         read_result = []
         if size < 0:
             for b in self._iterator:
-                read_result += b
+                read_result.append(b)
         else:
             for i in range(size):
                 try:
                     b = self._iterator.__next__()
                 except StopIteration:
                     break
-                read_result += b
+                read_result.append(b)
 
         return bytes(read_result)