diff --git a/pkg/lwip/include/arch/sys_arch.h b/pkg/lwip/include/arch/sys_arch.h
index 7ac430a30b495..31771aa10d9bc 100644
--- a/pkg/lwip/include/arch/sys_arch.h
+++ b/pkg/lwip/include/arch/sys_arch.h
@@ -100,13 +100,13 @@ typedef struct {
 
 static inline bool sys_mbox_valid(sys_mbox_t *mbox)
 {
-    return (mbox != NULL) && (mbox->mbox.cib.mask != 0);
+    return (mbox != NULL) && (mbox_size(&mbox->mbox) != 0);
 }
 
 static inline void sys_mbox_set_invalid(sys_mbox_t *mbox)
 {
     if (mbox != NULL) {
-        mbox->mbox.cib.mask = 0;
+        mbox_unset(&mbox->mbox);
     }
 }