diff --git a/gluon/globals.py b/gluon/globals.py
index 7091f40c..166a8f8b 100644
--- a/gluon/globals.py
+++ b/gluon/globals.py
@@ -626,6 +626,8 @@ class Response(Storage):
         return self.stream(stream, chunk_size=chunk_size, request=request)
 
     def json(self, data, default=None):
+        if 'Content-Type' not in self.headers:
+            self.headers['Content-Type'] = 'application/json'
         return json(data, default=default or custom_json)
 
     def xmlrpc(self, request, methods):