diff --git a/src/core/dhcp.c b/src/core/dhcp.c
index ecfe283b..fe85a7e8 100644
--- a/src/core/dhcp.c
+++ b/src/core/dhcp.c
@@ -76,7 +76,6 @@
 #include "lwip/ip_addr.h"
 #include "lwip/netif.h"
 #include "lwip/def.h"
-#include "lwip/sys.h"
 #include "lwip/dhcp.h"
 #include "lwip/autoip.h"
 #include "lwip/dns.h"
diff --git a/src/core/tcp_out.c b/src/core/tcp_out.c
index 86e09195..cf4ff76a 100644
--- a/src/core/tcp_out.c
+++ b/src/core/tcp_out.c
@@ -46,12 +46,14 @@
 #include "lwip/def.h"
 #include "lwip/mem.h"
 #include "lwip/memp.h"
-#include "lwip/sys.h"
 #include "lwip/ip_addr.h"
 #include "lwip/netif.h"
 #include "lwip/inet_chksum.h"
 #include "lwip/stats.h"
 #include "lwip/snmp.h"
+#if LWIP_TCP_TIMESTAMPS
+#include "lwip/sys.h"
+#endif
 
 #include <string.h>
 
@@ -810,7 +812,6 @@ tcp_enqueue_flags(struct tcp_pcb *pcb, u8_t flags)
 
   return ERR_OK;
 }
- 
 
 #if LWIP_TCP_TIMESTAMPS
 /* Build a timestamp option (12 bytes long) at the specified options pointer)
diff --git a/src/core/timers.c b/src/core/timers.c
index f0e92cce..37c04982 100644
--- a/src/core/timers.c
+++ b/src/core/timers.c
@@ -56,6 +56,7 @@
 #include "lwip/autoip.h"
 #include "lwip/igmp.h"
 #include "lwip/dns.h"
+#include "lwip/sys.h"
 
 
 /** The one and only timeout list */
diff --git a/src/include/lwip/tcp.h b/src/include/lwip/tcp.h
index 07dcd10e..60024257 100644
--- a/src/include/lwip/tcp.h
+++ b/src/include/lwip/tcp.h
@@ -36,7 +36,6 @@
 
 #if LWIP_TCP /* don't build if not configured for use in lwipopts.h */
 
-#include "lwip/sys.h"
 #include "lwip/mem.h"
 #include "lwip/pbuf.h"
 #include "lwip/ip.h"
diff --git a/src/include/lwip/tcp_impl.h b/src/include/lwip/tcp_impl.h
index b4feec0d..02697038 100644
--- a/src/include/lwip/tcp_impl.h
+++ b/src/include/lwip/tcp_impl.h
@@ -37,7 +37,6 @@
 #if LWIP_TCP /* don't build if not configured for use in lwipopts.h */
 
 #include "lwip/tcp.h"
-#include "lwip/sys.h"
 #include "lwip/mem.h"
 #include "lwip/pbuf.h"
 #include "lwip/ip.h"
diff --git a/src/include/lwip/timers.h b/src/include/lwip/timers.h
index fb92b4b4..04e78e0f 100644
--- a/src/include/lwip/timers.h
+++ b/src/include/lwip/timers.h
@@ -41,7 +41,9 @@
 #if LWIP_TIMERS
 
 #include "lwip/err.h"
+#if !NO_SYS
 #include "lwip/sys.h"
+#endif
 
 #ifdef __cplusplus
 extern "C" {
diff --git a/src/netif/ethernetif.c b/src/netif/ethernetif.c
index a5b7d990..8ec40be1 100644
--- a/src/netif/ethernetif.c
+++ b/src/netif/ethernetif.c
@@ -50,7 +50,6 @@
 #include "lwip/def.h"
 #include "lwip/mem.h"
 #include "lwip/pbuf.h"
-#include "lwip/sys.h"
 #include <lwip/stats.h>
 #include <lwip/snmp.h>
 #include "netif/etharp.h"
diff --git a/src/netif/slipif.c b/src/netif/slipif.c
index c19333dd..c908a1e4 100644
--- a/src/netif/slipif.c
+++ b/src/netif/slipif.c
@@ -49,10 +49,12 @@
 
 #include "lwip/def.h"
 #include "lwip/pbuf.h"
-#include "lwip/sys.h"
 #include "lwip/stats.h"
 #include "lwip/snmp.h"
 #include "lwip/sio.h"
+#if !NO_SYS
+#include "lwip/sys.h"
+#endif
 
 #define SLIP_BLOCK     1
 #define SLIP_DONTBLOCK 0
@@ -335,9 +337,11 @@ slipif_init(struct netif *netif)
    */
   NETIF_INIT_SNMP(netif, snmp_ifType_slip, 0);
 
+#if !NO_SYS
   /* Create a thread to poll the serial line. */
   sys_thread_new(SLIPIF_THREAD_NAME, slipif_loop_thread, netif,
     SLIPIF_THREAD_STACKSIZE, SLIPIF_THREAD_PRIO);
+#endif
   return ERR_OK;
 }