diff --git a/_locales/en/messages.json b/_locales/en/messages.json
index d8b8cb9f..721559ca 100644
--- a/_locales/en/messages.json
+++ b/_locales/en/messages.json
@@ -773,6 +773,15 @@
       }
     }
   },
+  "styleMetaErrorPreprocessor": {
+    "message": "Unsupported @preprocessor: $preprocessor$",
+    "description": "Error displayed when the value of @preprocessor is not supported",
+    "placeholders": {
+      "preprocessor": {
+        "content": "$1"
+      }
+    }
+  },
   "styleMetaErrorSelectValueMismatch": {
     "message": "Invalid @select: value doesn't exist in the list",
     "description": "Error displayed when the value of @select is invalid"
diff --git a/js/usercss.js b/js/usercss.js
index 1f4d88ce..e6e359bd 100644
--- a/js/usercss.js
+++ b/js/usercss.js
@@ -403,7 +403,7 @@ var usercss = (() => {
     let builder;
     if (preprocessor) {
       if (!BUILDER[preprocessor]) {
-        return Promise.reject(new Error(`Unsupported preprocessor: ${preprocessor}`));
+        return Promise.reject(chrome.i18n.getMessage('styleMetaErrorPreprocessor', preprocessor));
       }
       builder = BUILDER[preprocessor];
     } else {