diff --git a/tools/doc/generate.js b/tools/doc/generate.js
index 0ae413ea7f..53c9c5b5ee 100644
--- a/tools/doc/generate.js
+++ b/tools/doc/generate.js
@@ -89,6 +89,6 @@ function next(er, input) {
       break;
 
     default:
-      throw new Error('Invalid format: ' + format);
+      throw new Error(`Invalid format: ${format}`);
   }
 }