diff --git a/src/page.c b/src/page.c
index c0659395..3368bad1 100644
--- a/src/page.c
+++ b/src/page.c
@@ -621,7 +621,11 @@ static void mi_page_init(mi_heap_t* heap, mi_page_t* page, size_t block_size, mi
   page->keys[0] = _mi_heap_random_next(heap);
   page->keys[1] = _mi_heap_random_next(heap);
   #endif
+  #if MI_DEBUG > 0
+  page->is_zero = false; // ensure in debug mode we initialize with MI_DEBUG_UNINIT, see issue #501
+  #else
   page->is_zero = page->is_zero_init;
+  #endif
 
   mi_assert_internal(page->capacity == 0);
   mi_assert_internal(page->free == NULL);