diff --git a/mitmproxy/web/app.py b/mitmproxy/web/app.py
index c92ba4d30..5498c2d96 100644
--- a/mitmproxy/web/app.py
+++ b/mitmproxy/web/app.py
@@ -116,7 +116,7 @@ class RequestHandler(BasicAuth, tornado.web.RequestHandler):
     def json(self):
         if not self.request.headers.get("Content-Type").startswith("application/json"):
             return None
-        return json.loads(self.request.body)
+        return json.loads(self.request.body.decode())
 
     @property
     def state(self):