diff --git a/src/core/init.c b/src/core/init.c
index 328b3ca5..c53dbbc2 100644
--- a/src/core/init.c
+++ b/src/core/init.c
@@ -96,6 +96,9 @@
 #if (LWIP_IGMP && (MEMP_NUM_IGMP_GROUP<=1))
   #error "If you want to use IGMP, you have to define MEMP_NUM_IGMP_GROUP>1 in your lwipopts.h"
 #endif
+#if (PPP_SUPPORT && (NO_SYS==1))
+  #error "If you want to use PPP, you have to define NO_SYS=0 in your lwipopts.h"
+#endif 
 #if ((LWIP_SOCKET || LWIP_NETCONN) && (NO_SYS==1))
   #error "If you want to use Sequential API, you have to define NO_SYS=0 in your lwipopts.h"
 #endif
diff --git a/src/netif/slipif.c b/src/netif/slipif.c
index 162d6a8a..d7604126 100644
--- a/src/netif/slipif.c
+++ b/src/netif/slipif.c
@@ -204,6 +204,7 @@ slipif_input(struct netif *netif)
   return NULL;
 }
 
+#if !NO_SYS
 /**
  * The SLIP input thread.
  *
@@ -227,6 +228,7 @@ slipif_loop(void *nf)
     }
   }
 }
+#endif /* !NO_SYS */
 
 /**
  * SLIP netif initialization