diff --git a/include/mimalloc-internal.h b/include/mimalloc-internal.h
index ca9e9c41..9fa37108 100644
--- a/include/mimalloc-internal.h
+++ b/include/mimalloc-internal.h
@@ -217,6 +217,12 @@ static inline bool _mi_is_power_of_two(uintptr_t x) {
   return ((x & (x - 1)) == 0);
 }
 
+// Is a pointer aligned?
+static inline bool _mi_is_aligned(void* p, size_t alignment) {
+  mi_assert_internal(alignment != 0);
+  return (((uintptr_t)p % alignment) == 0);
+}
+
 // Align upwards
 static inline uintptr_t _mi_align_up(uintptr_t sz, size_t alignment) {
   mi_assert_internal(alignment != 0);
diff --git a/src/alloc.c b/src/alloc.c
index 30235944..19844849 100644
--- a/src/alloc.c
+++ b/src/alloc.c
@@ -633,7 +633,9 @@ void* _mi_heap_realloc_zero(mi_heap_t* heap, void* p, size_t newsize, bool zero)
       memset((uint8_t*)newp + start, 0, newsize - start);
     }
     if mi_likely(p != NULL) {
-      _mi_memcpy_aligned(newp, p, (newsize > size ? size : newsize));
+      if mi_likely(_mi_is_aligned(p, sizeof(uintptr_t))) {  // a client may pass in an arbitrary pointer `p`..
+        _mi_memcpy_aligned(newp, p, (newsize > size ? size : newsize));
+      }
       mi_free(p); // only free the original pointer if successful
     }
   }