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 += `${firstLine + i}
`;
- content += formatLine(line, firstLine + i, mark, options);
+ numbers += `${Number(firstLine) + i}
`;
+ content += formatLine(line, Number(firstLine) + i, mark, options);
}
let result = `