diff --git a/lib/format.js b/lib/format.js
index a85c3ef7..e138831d 100644
--- a/lib/format.js
+++ b/lib/format.js
@@ -74,7 +74,7 @@ export function stringToColor(str) {
   let color = '#';
   for (let i = 0; i < 3; i++) {
     let value = (hash >> (i * 8)) & 0xff;
-    color += ('00' + value.toString(16)).substring(-2);
+    color += ('00' + value.toString(16)).slice(-2);
   }
   return color;
 }