diff --git a/core/include/mbox.h b/core/include/mbox.h
index 62a439271119..121b9ae65a59 100644
--- a/core/include/mbox.h
+++ b/core/include/mbox.h
@@ -185,6 +185,17 @@ static inline size_t mbox_avail(mbox_t *mbox)
     return cib_avail(&mbox->cib);
 }
 
+/**
+ * @brief   Unset's the mbox, effectively deinitializing and invalidating it.
+ *
+ * @param[in] mbox  ptr to mailbox to operate on
+ */
+static inline void mbox_unset(mbox_t *mbox)
+{
+    mbox->msg_array = NULL;
+    mbox->cib.mask = 0;
+}
+
 #ifdef __cplusplus
 }
 #endif
diff --git a/pkg/lwip/include/arch/sys_arch.h b/pkg/lwip/include/arch/sys_arch.h
index c9dda62357c0..64c38a3441a7 100644
--- a/pkg/lwip/include/arch/sys_arch.h
+++ b/pkg/lwip/include/arch/sys_arch.h
@@ -101,13 +101,13 @@ 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);
     }
 }