diff --git a/adb/sign_pythonrsa.py b/adb/sign_pythonrsa.py
index 5c00bb2..fcf0725 100644
--- a/adb/sign_pythonrsa.py
+++ b/adb/sign_pythonrsa.py
@@ -24,7 +24,7 @@
 # hashing algo for this.
 class _Accum(object):
   def __init__(self):
-    self._buf = ''
+    self._buf = b''
   def update(self, msg):
     self._buf += msg
   def digest(self):