diff --git a/pbkdf2.py b/pbkdf2.py
index b7a7dd4..29455db 100644
--- a/pbkdf2.py
+++ b/pbkdf2.py
@@ -72,7 +72,7 @@ def _pseudorandom(x, mac=mac):
         rv = u = _pseudorandom(salt + _pack_int(block))
         for i in xrange(iterations - 1):
             u = _pseudorandom(''.join(map(chr, u)))
-            rv = starmap(xor, izip(rv, u))
+            rv = list(starmap(xor, izip(rv, u)))
         buf.extend(rv)
     return ''.join(map(chr, buf))[:keylen]