diff --git a/src/heap/cppgc-js/unified-heap-marking-state-inl.h b/src/heap/cppgc-js/unified-heap-marking-state-inl.h
index 888bbd0c43..fd4d7cf97f 100644
--- a/src/heap/cppgc-js/unified-heap-marking-state-inl.h
+++ b/src/heap/cppgc-js/unified-heap-marking-state-inl.h
@@ -46,6 +46,7 @@ void UnifiedHeapMarkingState::MarkAndPush(
     return;
   }
   HeapObject heap_object = HeapObject::cast(object);
+  if (heap_object.InReadOnlySpace()) return;
   if (marking_state_->WhiteToGrey(heap_object)) {
     local_marking_worklist_->Push(heap_object);
   }
diff --git a/src/heap/marking-barrier-inl.h b/src/heap/marking-barrier-inl.h
index 37c31c515b..220a9a8aa8 100644
--- a/src/heap/marking-barrier-inl.h
+++ b/src/heap/marking-barrier-inl.h
@@ -72,6 +72,7 @@ void MarkingBarrier::MarkValueShared(HeapObject value) {
 }
 
 void MarkingBarrier::MarkValueLocal(HeapObject value) {
+  DCHECK(!value.InReadOnlySpace());
   if (is_minor()) {
     // We do not need to insert into RememberedSet<OLD_TO_NEW> here because the
     // C++ marking barrier already does this for us.
diff --git a/src/heap/marking-barrier.cc b/src/heap/marking-barrier.cc
index 5a31efd1d1..9ce722b47a 100644
--- a/src/heap/marking-barrier.cc
+++ b/src/heap/marking-barrier.cc
@@ -60,7 +60,7 @@ void MarkingBarrier::WriteWithoutHost(HeapObject value) {
       return;
     }
   }
-
+  if (value.InReadOnlySpace()) return;
   MarkValueLocal(value);
 }