diff --git a/lib/highlight.js b/lib/highlight.js
index d3a4a0bd..106f222d 100644
--- a/lib/highlight.js
+++ b/lib/highlight.js
@@ -36,8 +36,8 @@ function highlightUtil(str, options = {}) {
   for (let i = 0, len = lines.length; i < len; i++) {
     let line = lines[i];
     if (tab) line = replaceTabs(line, tab);
-    numbers += `<span class="line">${firstLine + i}</span><br>`;
-    content += formatLine(line, firstLine + i, mark, options);
+    numbers += `<span class="line">${Number(firstLine) + i}</span><br>`;
+    content += formatLine(line, Number(firstLine) + i, mark, options);
   }
 
   let result = `<figure class="highlight${data.language ? ` ${data.language}` : ''}">`;