--- a/toolkit/components/extensions/Schemas.jsm
+++ b/toolkit/components/extensions/Schemas.jsm
@@ -1182,17 +1182,17 @@ class Type extends Entry {
   // normalize. Subclasses can choose to use it or not.
   normalizeBase(type, value, context) {
     if (this.checkBaseType(getValueBaseType(value))) {
       this.checkDeprecated(context, value);
       return {value: this.preprocess(value, context)};
     }
 
     let choice;
-    if (/^[aeiou]/.test(type)) {
+    if ("aeiou".includes(type[0])) {
       choice = `be an ${type} value`;
     } else {
       choice = `be a ${type} value`;
     }
 
     return context.error(() => `Expected ${type} instead of ${JSON.stringify(value)}`,
                          choice);
   }