diff --git a/include/mimalloc-internal.h b/include/mimalloc-internal.h
index 16e178b0..26fd36b2 100644
--- a/include/mimalloc-internal.h
+++ b/include/mimalloc-internal.h
@@ -148,6 +148,10 @@ void        _mi_block_zero_init(const mi_page_t* page, void* p, size_t size);
 bool        _mi_page_is_valid(mi_page_t* page);
 #endif
 
+#if MI_DEBUG >= 2
+void        _mi_debug_uninit_fill(void* p, size_t size);
+#endif
+
 
 // ------------------------------------------------------
 // Branches
diff --git a/src/alloc-aligned.c b/src/alloc-aligned.c
index 1c63be9b..ae493f46 100644
--- a/src/alloc-aligned.c
+++ b/src/alloc-aligned.c
@@ -83,6 +83,11 @@ static void* mi_heap_malloc_init_aligned_at(mi_heap_t* const heap, const size_t
       mi_assert_internal(p != NULL);
       mi_assert_internal(((uintptr_t)p + offset) % alignment == 0);
       if (init == MI_ALLOC_ZERO_INIT) { _mi_block_zero_init(page, p, size); }
+      else if (init == MI_ALLOC_UNINIT) {
+      #if MI_DEBUG >= 2
+        _mi_debug_uninit_fill(p, size);
+      #endif
+      }
       return p;
     }
   }
diff --git a/src/alloc.c b/src/alloc.c
index a92115ea..81b66869 100644
--- a/src/alloc.c
+++ b/src/alloc.c
@@ -37,9 +37,7 @@ extern inline void* _mi_page_malloc(mi_heap_t* heap, mi_page_t* page, size_t siz
   page->free = mi_block_next(page, block);
   mi_assert_internal(page->free == NULL || _mi_ptr_page(page->free) == page);
 
-#if (MI_DEBUG>0)
-  if (!page->is_zero) { memset(block, MI_DEBUG_UNINIT, size); }
-#elif (MI_SECURE!=0)
+#if (MI_SECURE!=0)
   block->next = 0;  // don't leak internal data
 #endif
 
@@ -90,6 +88,10 @@ static inline mi_decl_restrict void* _mi_heap_malloc_small_init(mi_heap_t* heap,
   #endif
   if (init == MI_ALLOC_ZERO_INIT && p != NULL) {
     _mi_block_zero_init(_mi_ptr_page(p),p,size);  // todo: can we avoid getting the page again?
+  } else if (init == MI_ALLOC_UNINIT && p != NULL) {
+    #if MI_DEBUG >= 2
+    _mi_debug_uninit_fill(p, size);
+    #endif
   }
   return p;
 }
@@ -120,6 +122,10 @@ extern inline mi_decl_restrict void* _mi_heap_malloc_init(mi_heap_t* heap, size_
     #endif
     if (init == MI_ALLOC_ZERO_INIT && p != NULL) {
       _mi_block_zero_init(_mi_ptr_page(p),p,size);  // todo: can we avoid getting the page again?
+    } else if (init == MI_ALLOC_UNINIT && p != NULL) {
+      #if MI_DEBUG >= 2
+      _mi_debug_uninit_fill(p, size);
+      #endif
     }
     return p;
   }
@@ -152,6 +158,12 @@ void _mi_block_zero_init(const mi_page_t* page, void* p, size_t size) {
   }
 }
 
+#if MI_DEBUG >= 2
+void _mi_debug_uninit_fill(void* p, size_t size) {
+  memset(p, MI_DEBUG_UNINIT, size);
+}
+#endif
+
 // zero initialized small block
 mi_decl_restrict void* mi_zalloc_small(size_t size) mi_attr_noexcept {
   return _mi_heap_malloc_small_init(mi_get_default_heap(), size, MI_ALLOC_ZERO_INIT);