diff --git a/src/include/lwip/debug.h b/src/include/lwip/debug.h
index 8209860a..73482aad 100644
--- a/src/include/lwip/debug.h
+++ b/src/include/lwip/debug.h
@@ -82,6 +82,10 @@
 #define SIO_FIFO_DEBUG   0
 #endif
 
+#ifndef SLIP_DEBUG
+#define SLIP_DEBUG        0
+#endif
+
 #ifndef PPP_DEBUG
 #define PPP_DEBUG        0
 #endif