diff --git a/packages/react-scripts/scripts/start.js b/packages/react-scripts/scripts/start.js
index 0c0c41a30a4..443cd449a3d 100644
--- a/packages/react-scripts/scripts/start.js
+++ b/packages/react-scripts/scripts/start.js
@@ -43,7 +43,7 @@ if (!checkRequiredFiles([paths.appHtml, paths.appIndexJs])) {
 }
 
 // Tools like Cloud9 rely on this.
-var DEFAULT_PORT = process.env.PORT || 3000;
+var DEFAULT_PORT = parseInt(process.env.PORT, 10) || 3000;
 var compiler;
 var handleCompile;