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 );
 
diff --git a/library/ssl_tls.c b/library/ssl_tls.c
index 7f82fa0..7cb6258 100644
--- a/library/ssl_tls.c
+++ b/library/ssl_tls.c
@@ -1770,7 +1770,7 @@
 
 void ssl_set_bio( ssl_context *ssl,
             int (*f_recv)(void *, unsigned char *, size_t), void *p_recv,
-            int (*f_send)(void *, unsigned char *, size_t), void *p_send )
+            int (*f_send)(void *, const unsigned char *, size_t), void *p_send )
 {
     ssl->f_recv     = f_recv;
     ssl->f_send     = f_send;