diff --git a/src/heap/heap-write-barrier-inl.h b/src/heap/heap-write-barrier-inl.h
index 7b1438e6a3..8fec301fe7 100644
--- a/src/heap/heap-write-barrier-inl.h
+++ b/src/heap/heap-write-barrier-inl.h
@@ -61,6 +61,7 @@ struct MemoryChunk {
 
   V8_INLINE static heap_internals::MemoryChunk* FromHeapObject(
       HeapObject object) {
+    DCHECK(!V8_ENABLE_THIRD_PARTY_HEAP_BOOL);
     return reinterpret_cast<MemoryChunk*>(object.ptr() & ~kPageAlignmentMask);
   }
 
diff --git a/src/heap/spaces.h b/src/heap/spaces.h
index 29a563afbc..f8b1cba5f8 100644
--- a/src/heap/spaces.h
+++ b/src/heap/spaces.h
@@ -641,10 +641,12 @@ class MemoryChunk : public BasicMemoryChunk {
 
   // Only works if the pointer is in the first kPageSize of the MemoryChunk.
   static MemoryChunk* FromAddress(Address a) {
+    DCHECK(!V8_ENABLE_THIRD_PARTY_HEAP_BOOL);
     return reinterpret_cast<MemoryChunk*>(BaseAddress(a));
   }
   // Only works if the object is in the first kPageSize of the MemoryChunk.
   static MemoryChunk* FromHeapObject(HeapObject o) {
+    DCHECK(!V8_ENABLE_THIRD_PARTY_HEAP_BOOL);
     return reinterpret_cast<MemoryChunk*>(BaseAddress(o.ptr()));
   }