diff --git a/Userland/Libraries/LibC/malloc.cpp b/Userland/Libraries/LibC/malloc.cpp
index b9afd147f44529..f67dc0b4f362bd 100644
--- a/Userland/Libraries/LibC/malloc.cpp
+++ b/Userland/Libraries/LibC/malloc.cpp
@@ -528,7 +528,7 @@ size_t malloc_size(void const* ptr)
     auto* header = (const CommonHeader*)page_base;
     auto size = header->m_size;
     if (header->m_magic == MAGIC_BIGALLOC_HEADER)
-        size -= sizeof(CommonHeader);
+        size -= sizeof(BigAllocationBlock);
     else
         VERIFY(header->m_magic == MAGIC_PAGE_HEADER);
     return size;
diff --git a/Userland/Libraries/LibC/mallocdefs.h b/Userland/Libraries/LibC/mallocdefs.h
index ca7cf9a30f73e7..63fd0071f35204 100644
--- a/Userland/Libraries/LibC/mallocdefs.h
+++ b/Userland/Libraries/LibC/mallocdefs.h
@@ -46,7 +46,7 @@ struct BigAllocationBlock : public CommonHeader {
         m_magic = MAGIC_BIGALLOC_HEADER;
         m_size = size;
     }
-    unsigned char* m_slot[0];
+    alignas(16) unsigned char* m_slot[0];
 };
 
 struct FreelistEntry {