diff --git a/src/include/lwip/opt.h b/src/include/lwip/opt.h
index ea0ea2a8..59c1754c 100644
--- a/src/include/lwip/opt.h
+++ b/src/include/lwip/opt.h
@@ -61,6 +61,9 @@
 #define SYS_ARCH_UNPROTECT(lock)
 #endif
 
+#ifndef NO_SYS
+#define NO_SYS 0
+#endif
 /* ---------- Memory options ---------- */
 /* MEM_ALIGNMENT: should be set to the alignment of the CPU for which
    lwIP is compiled. 4 byte alignment -> define MEM_ALIGNMENT to 4, 2