diff --git a/js/usercss.js b/js/usercss.js
index 61a7a66b..11e1dac0 100644
--- a/js/usercss.js
+++ b/js/usercss.js
@@ -579,7 +579,7 @@ var usercss = (() => {
       worker.instance = new Worker('/vendor-overwrites/csslint/csslint-worker.js');
       worker.queue = [];
       worker.instance.onmessage = ({data}) => {
-        worker.queue.shift().resolve(data);
+        worker.queue.shift().resolve(data.__ERROR__ ? Promise.reject(data.__ERROR__) : data);
         if (worker.queue.length) {
           worker.instance.postMessage(worker.queue[0].message);
         }
diff --git a/manage/config-dialog.js b/manage/config-dialog.js
index ec38aaab..08da4c69 100644
--- a/manage/config-dialog.js
+++ b/manage/config-dialog.js
@@ -30,11 +30,7 @@ function configDialog(style) {
       {textContent: t('confirmClose'), dataset: {cmd: 'close'}},
     ],
     onshow,
-  }).then(() => {
-    document.body.style.minWidth = '';
-    document.body.style.minHeight = '';
-    colorpicker.hide();
-  });
+  }).then(onhide);
 
   function getInitialValues(source) {
     const data = {};
@@ -69,6 +65,12 @@ function configDialog(style) {
     updateButtons();
   }
 
+  function onhide() {
+    document.body.style.minWidth = '';
+    document.body.style.minHeight = '';
+    colorpicker.hide();
+  }
+
   function onchange({target}) {
     // invoked after element's own onchange so 'va' contains the updated value
     const va = target.va;
@@ -129,6 +131,7 @@ function configDialog(style) {
       }
     }
     if (invalid.length) {
+      onhide();
       messageBox.alert([
         $create('div', {style: 'max-width: 34em'}, t('usercssConfigIncomplete')),
         $create('ol', {style: 'text-align: left'},
@@ -136,11 +139,16 @@ function configDialog(style) {
             $create({tag: 'li', appendChild: msg}))),
       ]);
     }
-    return numValid && BG.usercssHelper.save(style).then(saved => {
-      varsInitial = getInitialValues(deepCopy(saved.usercssData.vars));
-      vars.forEach(va => onchange({target: va.input}));
-      updateButtons();
-    });
+    if (!numValid) {
+      return;
+    }
+    return BG.usercssHelper.save(style)
+      .then(saved => {
+        varsInitial = getInitialValues(deepCopy(saved.usercssData.vars));
+        vars.forEach(va => onchange({target: va.input}));
+        updateButtons();
+      })
+      .catch(errors => onhide() + messageBox.alert(Array.isArray(errors) ? errors.join('\n') : errors));
   }
 
   function useDefault() {
diff --git a/vendor-overwrites/csslint/csslint-worker.js b/vendor-overwrites/csslint/csslint-worker.js
index d73479f0..c76ae6cb 100644
--- a/vendor-overwrites/csslint/csslint-worker.js
+++ b/vendor-overwrites/csslint/csslint-worker.js
@@ -11008,19 +11008,19 @@ self.onmessage = ({data: {action = 'run', code, config}}) => {
       self.postMessage(CSSLint.getRules().map(rule => JSON.parse(JSON.stringify(rule))));
       return;
 
-    case 'run':
-      self.postMessage(CSSLint.verify(code, config).messages.map(m => {
-        // the functions are non-tranferable and we need only an id
-        m.rule = {id: m.rule.id};
-        return m;
-      }).filter(m => !m.message.includes('/*[[') && !m.message.includes(']]*/')));
+    case 'run': {
+      const results = CSSLint.verify(code, config).messages
+        .filter(m => !m.message.includes('/*[[') && !m.message.includes(']]*/'))
+        .map(m => Object.assign(m, {rule: {id: m.rule.id}}));
+      self.postMessage(results);
       return;
-
+    }
     case 'parse':
       if (!self.mozParser) {
         self.importScripts('/js/moz-parser.js');
       }
       mozParser.parse(code)
-        .then(sections => self.postMessage(sections));
+        .then(sections => self.postMessage(sections))
+        .catch(info => self.postMessage({__ERROR__: info}));
   }
 };