diff --git a/library/ssl_cli.c b/library/ssl_cli.c
index b3521d318..bb2afb262 100644
--- a/library/ssl_cli.c
+++ b/library/ssl_cli.c
@@ -1886,7 +1886,7 @@ static int ssl_write_client_key_exchange( ssl_context *ssl )
             ssl->out_msg[i++] = (unsigned char)( n      );
 
             ret = dhm_make_public( &ssl->handshake->dhm_ctx,
-                    mpi_size( &ssl->handshake->dhm_ctx.P ),
+                    (int) mpi_size( &ssl->handshake->dhm_ctx.P ),
                     &ssl->out_msg[i], n,
                     ssl->f_rng, ssl->p_rng );
             if( ret != 0 )