diff --git a/install-usercss/install-usercss.js b/install-usercss/install-usercss.js
index 663b40ec..755d3308 100644
--- a/install-usercss/install-usercss.js
+++ b/install-usercss/install-usercss.js
@@ -242,7 +242,7 @@
     const contents = Array.isArray(err) ?
       [$create('pre', err.join('\n'))] :
       [err && err.message && $create('pre', err.message) || err || 'Unknown error'];
-    if (Number.isInteger(err.index)) {
+    if (Number.isInteger(err.index) && typeof contents[0] === 'string') {
       const pos = cm.posFromIndex(err.index);
       contents[0] = `${pos.line + 1}:${pos.ch + 1} ` + contents[0];
       contents.push($create('pre', drawLinePointer(pos)));