diff --git a/src/alloc.c b/src/alloc.c
index f9bbf66c..ecbd58fa 100644
--- a/src/alloc.c
+++ b/src/alloc.c
@@ -97,8 +97,8 @@ void _mi_block_zero_init(void* p, size_t size) {
     if (page->flags.is_zero) {
       ((mi_block_t*)p)->next = 0;
       #if MI_DEBUG>0
-      for (size_t i = 0; i < size; i++) { 
-        if (((uint8_t*)p)[i] != 0) {
+      for (size_t i = 0; i < (page->block_size/sizeof(uintptr_t)); i++) { 
+        if (((uintptr_t*)p)[i] != 0) {
           _mi_assert_fail("page not zero", __FILE__, __LINE__, "_mi_block_zero_init");
         }
       }
diff --git a/src/os.c b/src/os.c
index 93d84365..9b5aac35 100644
--- a/src/os.c
+++ b/src/os.c
@@ -601,8 +601,9 @@ static bool mi_os_commitx(void* addr, size_t size, bool commit, bool conservativ
   }
 
   #if defined(_WIN32)
-  *is_zero = true;
   if (commit) {
+    // if the memory was already committed, the call succeeds but it is not zero'd
+    // *is_zero = true;
     void* p = VirtualAlloc(start, csize, MEM_COMMIT, PAGE_READWRITE);
     err = (p == start ? 0 : GetLastError());
   }
diff --git a/src/page.c b/src/page.c
index f0f43960..ee8a3997 100644
--- a/src/page.c
+++ b/src/page.c
@@ -81,13 +81,17 @@ static bool mi_page_is_valid_init(mi_page_t* page) {
   mi_assert_internal(mi_page_list_is_valid(page,page->free));
   mi_assert_internal(mi_page_list_is_valid(page,page->local_free));
 
+  #if 0
   if (page->flags.is_zero) {
     for(mi_block_t* block = page->free; block != NULL; mi_block_next(page,block)) {
-      for (size_t i = sizeof(mi_block_t); i < page->block_size; i++) {
-        mi_assert_internal(0 == *((uint8_t*)block + i));
+      for (size_t i = 1; i < (page->block_size/sizeof(uintptr_t)); i++) {
+        if (((uintptr_t*)block)[i] != 0) {
+          return false;
+        }
       }
     }
   }
+  #endif
 
   mi_block_t* tfree = mi_tf_block(page->thread_free);
   mi_assert_internal(mi_page_list_is_valid(page, tfree));
@@ -558,7 +562,9 @@ static void mi_page_extend_free(mi_heap_t* heap, mi_page_t* page, mi_stats_t* st
   _mi_stat_increase(&stats->page_committed, extend * page->block_size);
 
   // extension into zero initialized memory preserves the zero'd free list
-  if (!page->is_zero_init) page->flags.is_zero = false;
+  if (!page->is_zero_init) {
+    page->flags.is_zero = false;
+  }
   mi_assert_expensive(mi_page_is_valid_init(page));
 }
 
@@ -577,7 +583,7 @@ static void mi_page_init(mi_heap_t* heap, mi_page_t* page, size_t block_size, mi
   #if MI_SECURE
   page->cookie = _mi_heap_random(heap) | 1;
   #endif
-  page->flags.is_zero =  page->is_zero_init;
+  page->flags.is_zero = page->is_zero_init;
 
   mi_assert_internal(page->capacity == 0);
   mi_assert_internal(page->free == NULL);