diff --git a/library/ssl_misc.h b/library/ssl_misc.h
index b74b0d9..598c3ef 100644
--- a/library/ssl_misc.h
+++ b/library/ssl_misc.h
@@ -1756,7 +1756,7 @@
 #if defined(MBEDTLS_KEY_EXCHANGE_WITH_CERT_ENABLED)
 int mbedtls_ssl_write_supported_groups_ext( mbedtls_ssl_context *ssl,
                                             unsigned char *buf,
-                                            unsigned char *end,
+                                            const unsigned char *end,
                                             size_t *out_len );
 
 #endif /* MBEDTLS_KEY_EXCHANGE_WITH_CERT_ENABLED */
diff --git a/library/ssl_tls.c b/library/ssl_tls.c
index 11d3c41..bab83ea 100644
--- a/library/ssl_tls.c
+++ b/library/ssl_tls.c
@@ -7236,7 +7236,7 @@
 
 int mbedtls_ssl_write_supported_groups_ext( mbedtls_ssl_context *ssl,
                                             unsigned char *buf,
-                                            unsigned char *end,
+                                            const unsigned char *end,
                                             size_t *out_len )
 {
     unsigned char *p = buf ;