diff --git a/lib/gnutls_extensions.c b/lib/gnutls_extensions.c
index 2ec5c15..99238a0 100644
--- a/lib/gnutls_extensions.c
+++ b/lib/gnutls_extensions.c
@@ -297,6 +297,7 @@ _gnutls_gen_extensions (gnutls_session_t session, gnutls_buffer_st * extdata,
   
   if ( size > 0)
     _gnutls_write_uint16(size, &extdata->data[pos]);
+  else if (size == 0) extdata->length = 0;
 
   return size;
 }