diff --git a/include/mimalloc-internal.h b/include/mimalloc-internal.h
index 2ef0839e..de77f3b5 100644
--- a/include/mimalloc-internal.h
+++ b/include/mimalloc-internal.h
@@ -20,7 +20,6 @@ terms of the MIT license. A copy of the license can be found in the file
 #define mi_trace_message(...)  
 #endif
 
-
 // "options.c"
 void       _mi_fputs(mi_output_fun* out, const char* prefix, const char* message);
 void       _mi_fprintf(mi_output_fun* out, const char* fmt, ...);
@@ -101,7 +100,7 @@ void*       _mi_heap_malloc_zero(mi_heap_t* heap, size_t size, bool zero);
 void*       _mi_heap_realloc_zero(mi_heap_t* heap, void* p, size_t newsize, bool zero);
 mi_block_t* _mi_page_ptr_unalign(const mi_segment_t* segment, const mi_page_t* page, const void* p);
 bool        _mi_free_delayed_block(mi_block_t* block);
-void        _mi_block_zero_init(void* p, size_t size);
+void        _mi_block_zero_init(const mi_page_t* page, void* p, size_t size);
 
 #if MI_DEBUG>1
 bool        _mi_page_is_valid(mi_page_t* page);
@@ -182,6 +181,14 @@ static inline uintptr_t _mi_align_up(uintptr_t sz, size_t alignment) {
   }
 }
 
+// Is memory zero initialized?
+static inline bool mi_mem_is_zero(void* p, size_t size) {
+  for (size_t i = 0; i < size; i++) {
+    if (((uint8_t*)p)[i] != 0) return false;
+  }
+  return true;
+}
+
 // Align a byte size to a size in _machine words_,
 // i.e. byte size == `wsize*sizeof(void*)`.
 static inline size_t _mi_wsize_from_size(size_t size) {
@@ -189,6 +196,11 @@ static inline size_t _mi_wsize_from_size(size_t size) {
   return (size + sizeof(uintptr_t) - 1) / sizeof(uintptr_t);
 }
 
+
+/* -----------------------------------------------------------
+  The thread local default heap
+----------------------------------------------------------- */
+
 extern const mi_heap_t _mi_heap_empty;  // read-only empty heap, initial value of the thread local default heap
 extern mi_heap_t _mi_heap_main;         // statically allocated main backing heap
 extern bool _mi_process_is_initialized;
@@ -220,6 +232,10 @@ static inline bool mi_heap_is_initialized(mi_heap_t* heap) {
   return (heap != &_mi_heap_empty);
 }
 
+/* -----------------------------------------------------------
+  Pages
+----------------------------------------------------------- */
+
 static inline mi_page_t* _mi_heap_get_free_small_page(mi_heap_t* heap, size_t size) {
   mi_assert_internal(size <= MI_SMALL_SIZE_MAX);
   return heap->pages_free_direct[_mi_wsize_from_size(size)];
@@ -230,7 +246,6 @@ static inline mi_page_t* _mi_get_free_small_page(size_t size) {
   return _mi_heap_get_free_small_page(mi_get_default_heap(), size);
 }
 
-
 // Segment that contains the pointer
 static inline mi_segment_t* _mi_ptr_segment(const void* p) {
   // mi_assert_internal(p != NULL);
diff --git a/src/alloc-aligned.c b/src/alloc-aligned.c
index a18ae7a5..c0ed2ece 100644
--- a/src/alloc-aligned.c
+++ b/src/alloc-aligned.c
@@ -33,7 +33,7 @@ static void* mi_heap_malloc_zero_aligned_at(mi_heap_t* heap, size_t size, size_t
       void* p = _mi_page_malloc(heap,page,size);
       mi_assert_internal(p != NULL);
       mi_assert_internal(((uintptr_t)p + offset) % alignment == 0);
-      if (zero) _mi_block_zero_init(p,size);
+      if (zero) _mi_block_zero_init(page,p,size);
       return p;
     }
   }
@@ -117,9 +117,16 @@ static void* mi_heap_realloc_zero_aligned_at(mi_heap_t* heap, void* p, size_t ne
     void* newp = mi_heap_malloc_aligned_at(heap,newsize,alignment,offset);
     if (newp != NULL) {
       if (zero && newsize > size) {
-        // also set last word in the previous allocation to zero to ensure any padding is zero-initialized
-        size_t start = (size >= sizeof(intptr_t) ? size - sizeof(intptr_t) : 0);
-        memset((uint8_t*)newp + start, 0, newsize - start);
+        const mi_page_t* page = _mi_ptr_page(newp);
+        if (page->flags.is_zero) {
+          // already zero initialized
+          mi_assert_expensive(mi_mem_is_zero(newp,newsize));
+        }
+        else {
+          // also set last word in the previous allocation to zero to ensure any padding is zero-initialized
+          size_t start = (size >= sizeof(intptr_t) ? size - sizeof(intptr_t) : 0);
+          memset((uint8_t*)newp + start, 0, newsize - start);
+        }
       }
       memcpy(newp, p, (newsize > size ? size : newsize));
       mi_free(p); // only free if successful
diff --git a/src/alloc.c b/src/alloc.c
index ecbd58fa..d8387178 100644
--- a/src/alloc.c
+++ b/src/alloc.c
@@ -89,31 +89,28 @@ extern inline void* mi_malloc(size_t size) mi_attr_noexcept {
   return mi_heap_malloc(mi_get_default_heap(), size);
 }
 
-void _mi_block_zero_init(void* p, size_t size) {
+void _mi_block_zero_init(const mi_page_t* page, void* p, size_t size) {
+  // note: we need to initialize the whole block to zero, not just size
+  // or the recalloc/rezalloc functions cannot safely expand in place (see issue #63)
+  UNUSED(size);
   mi_assert_internal(p != NULL);
-  // already zero initialized memory?
-  if (size > 4*sizeof(void*)) {  // don't bother for small sizes
-    mi_page_t* page = _mi_ptr_page(p);
-    if (page->flags.is_zero) {
-      ((mi_block_t*)p)->next = 0;
-      #if MI_DEBUG>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");
-        }
-      }
-      #endif
-      return; // and done
-    }
+  mi_assert_internal(size > 0 && page->block_size >= size);
+  mi_assert_internal(_mi_ptr_page(p)==page);
+  if (page->flags.is_zero) {
+    // already zero initialized memory?
+    ((mi_block_t*)p)->next = 0;  // clear the free list pointer
+    mi_assert_expensive(mi_mem_is_zero(p,page->block_size));
+  }
+  else {
+    // otherwise memset
+    memset(p, 0, page->block_size);
   }
-  // otherwise memset
-  memset(p, 0, size);
 }
 
 void* _mi_heap_malloc_zero(mi_heap_t* heap, size_t size, bool zero) {
   void* p = mi_heap_malloc(heap,size);
   if (zero && p != NULL) {
-    _mi_block_zero_init(p,size);
+    _mi_block_zero_init(_mi_ptr_page(p),p,size);  // todo: can we avoid getting the page again?
   }
   return p;
 }
diff --git a/src/page.c b/src/page.c
index ee8a3997..1288fd83 100644
--- a/src/page.c
+++ b/src/page.c
@@ -81,14 +81,10 @@ 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 MI_DEBUG>3 // generally too expensive to check this
   if (page->flags.is_zero) {
     for(mi_block_t* block = page->free; block != NULL; mi_block_next(page,block)) {
-      for (size_t i = 1; i < (page->block_size/sizeof(uintptr_t)); i++) {
-        if (((uintptr_t*)block)[i] != 0) {
-          return false;
-        }
-      }
+      mi_assert_expensive(mi_mem_is_zero(block + 1, page->block_size - sizeof(mi_block_t)));
     }
   }
   #endif