@@ -233,10 +233,11 @@ export class BuildStage extends StageBase<BuildStageHooks, IBuildStageProperties
   public static getOptionsFromStandardParameters(
     standardParameters: IBuildStageStandardParameters
   ): Omit<IBuildStageOptions, 'watchMode' | 'serveMode'> {
+    const localesParameterValues: readonly string[] = standardParameters.localesParameter.values;
     return {
       production: standardParameters.productionFlag.value,
       lite: standardParameters.liteFlag.value,
-      locales: standardParameters.localesParameter.values,
+      locales: localesParameterValues.length > 0 ? localesParameterValues : undefined,
       maxOldSpaceSize: standardParameters.maxOldSpaceSizeParameter.value,
       typescriptMaxWriteParallelism: standardParameters.typescriptMaxWriteParallelismParameter.value
     };
 
@@ -0,0 +1,10 @@
+{
+  "changes": [
+    {
+      "packageName": "@rushstack/heft",
+      "comment": "Fix an issue with the `locales` build property. The property is now undefined if no `--locale` parameters are specified.",
+      "type": "patch"
+    }
+  ],
+  "packageName": "@rushstack/heft"
+}