diff --git a/library/ssl_srv.c b/library/ssl_srv.c
index 07468546c0..ab6d2d5c03 100644
--- a/library/ssl_srv.c
+++ b/library/ssl_srv.c
@@ -2922,10 +2922,12 @@ static int ssl_prepare_server_key_exchange( mbedtls_ssl_context *ssl,
     (void) signature_len;
 #endif /* MBEDTLS_KEY_EXCHANGE_WITH_SERVER_SIGNATURE_ENABLED */
 
+#if defined(MBEDTLS_KEY_EXCHANGE_WITH_SERVER_SIGNATURE_ENABLED)
 #if defined(MBEDTLS_SSL_VARIABLE_BUFFER_LENGTH)
     size_t out_buf_len = ssl->out_buf_len - ( ssl->out_msg - ssl->out_buf );
 #else
     size_t out_buf_len = MBEDTLS_SSL_OUT_BUFFER_LEN - ( ssl->out_msg - ssl->out_buf );
+#endif
 #endif
 
     ssl->out_msglen = 4; /* header (type:1, length:3) to be written later */