diff --git a/src/netif/ppp/pppos.c b/src/netif/ppp/pppos.c
index d2aa97d4..006971d9 100644
--- a/src/netif/ppp/pppos.c
+++ b/src/netif/ppp/pppos.c
@@ -35,6 +35,7 @@
 #if PPP_SUPPORT && PPPOS_SUPPORT /* don't build if not configured for use in lwipopts.h */
 
 #include <string.h>
+#include <stddef.h>
 
 #include "lwip/err.h"
 #include "lwip/pbuf.h"
@@ -310,7 +311,7 @@ pppos_connect(ppp_pcb *ppp, void *ctx)
 
   ppp_link_start(ppp);
   /* reset PPPoS control block to its initial state */
-  memset(&pppos->last_xmit, 0, sizeof(pppos_pcb) - ( (char*)&((pppos_pcb*)0)->last_xmit - (char*)0 ) );
+  memset(&pppos->last_xmit, 0, sizeof(pppos_pcb) - offsetof(pppos_pcb, last_xmit));
 
   /*
    * Default the in and out accm so that escape and flag characters
@@ -344,7 +345,7 @@ pppos_listen(ppp_pcb *ppp, void *ctx)
 
   ppp_link_start(ppp);
   /* reset PPPoS control block to its initial state */
-  memset(&pppos->last_xmit, 0, sizeof(pppos_pcb) - ( (char*)&((pppos_pcb*)0)->last_xmit - (char*)0 ) );
+  memset(&pppos->last_xmit, 0, sizeof(pppos_pcb) - offsetof(pppos_pcb, last_xmit));
 
   /*
    * Default the in and out accm so that escape and flag characters