diff --git a/deploy/app.js b/deploy/app.js
index d753d3b1c5a0b4a4cbc6be2a919be24bd7d80823..b354a4b6839602e057a013fbb4feba81d74ecbcd 100644
--- a/deploy/app.js
+++ b/deploy/app.js
@@ -87,7 +87,7 @@ const { compressionMiddleware } = require('nomiseco')
 app.use(compressionMiddleware, express.static(PUBLIC_PATH))
 
 const jsonMiddleware = (req, res, next) => {
-  res.set('Content-Type', 'application/json')
+  if (!res.get('Content-Type')) res.set('Content-Type', 'application/json')
   next()
 }