diff --git a/fulfil_client/serialization.py b/fulfil_client/serialization.py
index 9a1f41d..5c97151 100644
--- a/fulfil_client/serialization.py
+++ b/fulfil_client/serialization.py
@@ -53,7 +53,7 @@ def __call__(self, dct):
 
 def _bytes_decoder(dct):
     cast = bytearray if bytes == str else bytes
-    return cast(base64.decodestring(dct['base64'].encode('utf-8')))
+    return cast(base64.b64decode(dct['base64'].encode('utf-8')))
 
 
 JSONDecoder.register('bytes', _bytes_decoder)