diff --git a/captcha/audio.py b/captcha/audio.py
index 1fc660f..31a300e 100644
--- a/captcha/audio.py
+++ b/captcha/audio.py
@@ -278,5 +278,5 @@ class AudioCaptcha(object):
         :param output: output destionation.
         """
         data = self.generate(chars)
-        with open(output, 'w') as f:
+        with open(output, 'wb') as f:
             return f.write(data)