diff --git a/lib/response.js b/lib/response.js
index <HASH>..<HASH> 100644
--- a/lib/response.js
+++ b/lib/response.js
@@ -71,7 +71,7 @@ class Response {
      * @param {string} type - MIME type of the response data
      */
     type(type) {
-        this.header('Content-Type', type);
+        this.header('content-type', type);
         return this;
     }
 }
refactor: Use lowercase key in content-type helper method