diff --git a/src/mem/pagemap.h b/src/mem/pagemap.h
index 94f5c1c..ce2aa5e 100644
--- a/src/mem/pagemap.h
+++ b/src/mem/pagemap.h
@@ -387,7 +387,7 @@ namespace snmalloc
      */
     size_t index_for_address(uintptr_t p)
     {
-      return reinterpret_cast<size_t>((p >> SHIFT) & (OS_PAGE_SIZE - 1));
+      return bits::align_down(static_cast<size_t>(p) >> SHIFT, OS_PAGE_SIZE);
     }
 
     /**