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;