diff --git a/awscrt/io.py b/awscrt/io.py
index 6e9473438..ce369c0f1 100644
--- a/awscrt/io.py
+++ b/awscrt/io.py
@@ -91,7 +91,7 @@ def byte_buf_null_terminate(buf):
         # I know this looks hacky. please don't change it
         # because appending bytes([0]) does not work in python 2.7
         # this works in both.
-        buf = buf + '\0'
+        buf = buf + b'\0'
     return buf