diff --git a/lib/utils.py b/lib/utils.py
index f204351..7bfb29f 100644
--- a/lib/utils.py
+++ b/lib/utils.py
@@ -40,6 +40,7 @@ def returns_json(handler):
 
   @functools.wraps(handler)
   def SerializeResult(self):
+    self.response.headers['Content-Type'] = 'application/json'
     json.dump(handler(self), self.response.out, default=EncodeJSON)
 
   return SerializeResult
diff --git a/static/cosmopolite.js b/static/cosmopolite.js
index 0bd8dbf..2749b57 100644
--- a/static/cosmopolite.js
+++ b/static/cosmopolite.js
@@ -950,6 +950,7 @@ Cosmopolite.prototype.sendRPCs_ = function(commands, opt_delay) {
       'load', this.onRPCResponse_.bind(this, xhr, retryAfterDelay, commands));
   xhr.addEventListener('error', retryAfterDelay.bind(null, commands));
   xhr.open('POST', this.urlPrefix_ + '/api');
+  xhr.setRequestHeader('Content-Type', 'application/json');
   xhr.responseType = 'json';
   xhr.send(JSON.stringify(request));
 };