diff --git a/src/annotator/index.js b/src/annotator/index.js
index 5417902109c..23e932a3e66 100644
--- a/src/annotator/index.js
+++ b/src/annotator/index.js
@@ -69,7 +69,7 @@ function init() {
 
     // Expose sidebar window reference for use by same-origin guest frames.
     window_.__hypothesis.sidebarWindow = sidebar.ready.then(
-      () => sidebar.iframe.contentWindow
+      () => [sidebar.iframe.contentWindow]
     );
   }
 
@@ -93,7 +93,7 @@ function init() {
 
   if (sidebarWindow) {
     const sidebarOrigin = new URL(sidebarLinkElement.href).origin;
-    sidebarWindow.then(frame =>
+    sidebarWindow.then(([frame]) =>
       guest.crossframe.connectToSidebar(frame, sidebarOrigin)
     );
   } else {
diff --git a/src/types/annotator.js b/src/types/annotator.js
index cbb9374647d..f9383acd1dd 100644
--- a/src/types/annotator.js
+++ b/src/types/annotator.js
@@ -143,9 +143,14 @@
  * @prop {import('./pdfjs').PDFViewerApplication} [PDFViewerApplication] -
  *   PDF.js entry point. If set, triggers loading of PDF rather than HTML integration.
  * @prop {object} [__hypothesis] - Internal data related to supporting guests in iframes
- *   @prop {Promise<Window>} [__hypothesis.sidebarWindow] -
+ *   @prop {Promise<[Window]>} [__hypothesis.sidebarWindow] -
  *     The sidebar window that is active in this frame. This resolves once the sidebar
  *     application has started and is ready to connect to guests.
+ *
+ *     The `Window` object is wrapped in an array to avoid issues with
+ *     combining promises with cross-origin objects in old browsers
+ *     (eg. Safari 11, Chrome <= 63) which trigger an exception when trying to
+ *     test if the object has a `then` method. See https://github.com/whatwg/dom/issues/536.
  */
 
 /**