diff --git a/src/main.js b/src/main.js
index ddd7e8d..8ea41e3 100644
--- a/src/main.js
+++ b/src/main.js
@@ -100,13 +100,13 @@ function start() {
         const el = findAnchorElement();
         if (el) {
           const rect = el.getBoundingClientRect();
-          dialog.setMode('anchored', { canAnchor: true });
-          dialog._positionAnchored(rect);
-          // observe
-          const ro = new ResizeObserver(() => {
-            if (prefs.mode === 'anchored') dialog._positionAnchored(el.getBoundingClientRect());
-          });
-          ro.observe(el);
+          dialog.setMode('anchored', { canAnchor: true, rect: rect });
+          if (typeof ResizeObserver !== 'undefined') {
+            const ro = new ResizeObserver(function () {
+              if (prefs.mode === 'anchored') dialog._positionAnchored(el.getBoundingClientRect());
+            });
+            ro.observe(el);
+          }
           anchorError = null;
           return;
         }
diff --git a/src/ui.js b/src/ui.js
index fee5988..0744ebd 100644
--- a/src/ui.js
+++ b/src/ui.js
@@ -135,6 +135,7 @@ export class Dialog {
   }
 
   _positionAnchored(rect) {
+    if (!rect) return;  // defensive: setMode may be called without a rect
     const dialogRect = this.root.getBoundingClientRect();
     let top = rect.top - dialogRect.height - 8;
     if (top < 8) top = 20;