diff --git a/serial/serialutil.py b/serial/serialutil.py
index 1bd0f0c..0844ecd 100644
--- a/serial/serialutil.py
+++ b/serial/serialutil.py
@@ -33,7 +33,11 @@
     else:
         b = bytearray()
         for item in seq:
-            b.append(item)  # this one handles int and str for our emulation and ints for Python 3.x
+            # this one handles int and bytes in Python 2.7
+            # add conversion in case of Python 3.x
+            if isinstance(item, bytes):
+                item = ord(item)
+            b.append(item)
         return bytes(b)
 
 # create control bytes