diff --git a/lib/middleware/cache.js b/lib/middleware/cache.js
index 4232e95060..f69275adee 100644
--- a/lib/middleware/cache.js
+++ b/lib/middleware/cache.js
@@ -45,7 +45,7 @@ module.exports = function(app, options = {}) {
             },
             set: async function(key, value, maxAge = 24 * 60 * 60) {
                 if (await this.get(key)) {
-                    logger.warn('repeated key: ', key, value);
+                    logger.warn(`repeated key: ${key}, ${value}`);
                 }
                 if (!value || value === 'undefined') {
                     value = '';