diff --git a/library/net.c b/library/net.c
index e1d91e4..fcfe4d7 100644
--- a/library/net.c
+++ b/library/net.c
@@ -317,7 +317,7 @@
 /*
  * Write at most 'len' characters
  */
-int net_send( void *ctx, unsigned char *buf, size_t len )
+int net_send( void *ctx, const unsigned char *buf, size_t len )
 {
     int ret = write( *((int *) ctx), buf, len );