diff --git a/src/heap/cppgc/object-start-bitmap.h b/src/heap/cppgc/object-start-bitmap.h
index 112ed653ac1..56aadd934de 100644
--- a/src/heap/cppgc/object-start-bitmap.h
+++ b/src/heap/cppgc/object-start-bitmap.h
@@ -129,6 +129,12 @@ HeapObjectHeader* ObjectStartBitmap::FindHeader(
     DCHECK_LT(0u, cell_index);
     byte = load<mode>(--cell_index);
   }
+  // Bandaid: if the scan reached cell 0 with no bit set, CountLeadingZeros(0)
+  // would yield object_start_number = SIZE_MAX and return page_base - 8 (i.e.
+  // the tail of the *adjacent* cage slot). This state is reachable on
+  // weak-memory hardware via a concurrent-marker / Member<Mixin> relaxed-store
+  // race. Crash safely instead of dereferencing groomed adjacent-page bytes.
+  CHECK_NE(0, byte);
   const int leading_zeroes = v8::base::bits::CountLeadingZeros(byte);
   object_start_number =
       (cell_index * kBitsPerCell) + (kBitsPerCell - 1) - leading_zeroes;