diff --git a/edit/codemirror-editing-hooks.js b/edit/codemirror-editing-hooks.js
index f2923894..3a8d436f 100644
--- a/edit/codemirror-editing-hooks.js
+++ b/edit/codemirror-editing-hooks.js
@@ -623,6 +623,10 @@ onDOMscriptReady('/codemirror.js').then(() => {
     }).then(() => {
       errors.classList.add('hidden');
     }).catch(err => {
+      if (err && editor && !Number.isNaN(err.index)) {
+        const pos = editors[0].posFromIndex(err.index);
+        err = `${pos.line}:${pos.ch} ${err}`;
+      }
       errors.classList.remove('hidden');
       errors.onclick = () => messageBox.alert(String(err), 'pre');
     });
diff --git a/js/usercss.js b/js/usercss.js
index cffa2204..46e213c7 100644
--- a/js/usercss.js
+++ b/js/usercss.js
@@ -450,8 +450,8 @@ var usercss = (() => {
     try {
       doParse();
     } catch (e) {
-      // grab additional info
-      e.index = metaIndex;
+      // the source code string offset
+      e.index = metaIndex + state.re.lastIndex;
       throw e;
     }