diff --git a/install-usercss/install-usercss.css b/install-usercss/install-usercss.css
index 1e0cdfef..0f1e2970 100644
--- a/install-usercss/install-usercss.css
+++ b/install-usercss/install-usercss.css
@@ -85,6 +85,7 @@ input:disabled + span {
   overflow-wrap: break-word;
   white-space: pre-wrap;
   margin: 1ex 0 0;
+  font-family: monospace;
   font-weight: normal;
   font-size: 80%;
 }
diff --git a/install-usercss/install-usercss.js b/install-usercss/install-usercss.js
index 2e1c3a36..e1f93e49 100644
--- a/install-usercss/install-usercss.js
+++ b/install-usercss/install-usercss.js
@@ -230,7 +230,7 @@
   function buildWarning(err) {
     const contents = Array.isArray(err) ?
       [$create('pre', err.join('\n'))] :
-      [err && err.message || err || 'Unknown error'];
+      [err && err.message && $create('pre', err.message) || err || 'Unknown error'];
     if (Number.isInteger(err.index)) {
       const pos = cm.posFromIndex(err.index);
       contents[0] = `${pos.line + 1}:${pos.ch + 1} ` + contents[0];