diff --git a/gatchat/ppp.h b/gatchat/ppp.h
index c7e6d246..9f68e5b9 100644
--- a/gatchat/ppp.h
+++ b/gatchat/ppp.h
@@ -101,7 +101,7 @@ void ppp_chap_process_packet(struct ppp_chap *chap, guint8 *new_packet);
 /* TUN / Network related functions */
 struct ppp_net *ppp_net_new(GAtPPP *ppp);
 const char *ppp_net_get_interface(struct ppp_net *net);
-void ppp_net_process_packet(struct ppp_net *net, guint8 *packet);
+void ppp_net_process_packet(struct ppp_net *net, const guint8 *packet);
 void ppp_net_free(struct ppp_net *net);
 gboolean ppp_net_set_mtu(struct ppp_net *net, guint16 mtu);
 
diff --git a/gatchat/ppp_net.c b/gatchat/ppp_net.c
index 14965c44..b4ff022f 100644
--- a/gatchat/ppp_net.c
+++ b/gatchat/ppp_net.c
@@ -73,7 +73,7 @@ gboolean ppp_net_set_mtu(struct ppp_net *net, guint16 mtu)
 	return (rc < 0) ? FALSE : TRUE;
 }
 
-void ppp_net_process_packet(struct ppp_net *net, guint8 *packet)
+void ppp_net_process_packet(struct ppp_net *net, const guint8 *packet)
 {
 	GError *error = NULL;
 	GIOStatus status;