diff --git a/third_party/blink/renderer/modules/xr/xr_frame.cc b/third_party/blink/renderer/modules/xr/xr_frame.cc
index 0d391336694c8..c24af96cfff6f 100644
--- a/third_party/blink/renderer/modules/xr/xr_frame.cc
+++ b/third_party/blink/renderer/modules/xr/xr_frame.cc
@@ -570,7 +570,17 @@ bool XRFrame::fillPoses(const HeapVector<Member<XRSpace>>& spaces,
   auto transforms_data = transforms->AsSpan();
   for (const auto& space : spaces) {
     auto current_transform = transforms_data.take_first<kFloatsPerTransform>();
-    if (const XRPose* pose = space->getPose(base_space)) {
+    const XRPose* pose = space->getPose(base_space);
+    // getPose() can reach XRBoundedReferenceSpace::EnsureUpdated() which
+    // synchronously dispatches a 'reset' event to author script. That script
+    // may transfer/detach |transforms|, leaving |transforms_data| (and
+    // |current_transform|) dangling. Re-check before every write.
+    if (transforms->IsDetached()) {
+      exception_state.ThrowTypeError(
+          "The transforms array was detached during fillPoses().");
+      return false;
+    }
+    if (pose) {
       current_transform.copy_from(pose->transform()->matrix()->AsSpan());
     } else {
       std::ranges::fill(current_transform, NAN);