diff --git a/src/service/viewer-impl.js b/src/service/viewer-impl.js
index 30ced57a4241..3a4b9428f2ef 100644
--- a/src/service/viewer-impl.js
+++ b/src/service/viewer-impl.js
@@ -806,6 +806,7 @@ export class Viewer {
    * the current page's URL. The trusted viewers are allowed to override this
    * value.
    * @return {!Promise<string>}
+   * @visibleForTesting
    */
   getViewerUrl() {
     return this.viewerUrl_;