diff --git a/lib/dev.js b/lib/dev.js
index 150971d58d..05828efa0d 100644
--- a/lib/dev.js
+++ b/lib/dev.js
@@ -102,7 +102,7 @@ module.exports = async function dev (sourceDir, cliOptions = {}) {
     host,
     dev: { logLevel: 'warn' },
     hot: {
-      port: port + 1,
+      port: parseInt(port) + 1,
       logLevel: 'error'
     },
     logLevel: 'error',