diff --git a/lwip/core/tcp_out.c b/lwip/core/tcp_out.c
index e2f8e9a..43fe3ca 100644
--- a/lwip/core/tcp_out.c
+++ b/lwip/core/tcp_out.c
@@ -448,7 +448,7 @@ tcp_write(struct tcp_pcb *pcb, const void *arg, u16_t len, u8_t apiflags)
     if (oversize > 0) {
       LWIP_ASSERT("inconsistent oversize vs. space", oversize_used <= space);
       seg = last_unsent;
-      oversize_used = oversize < len ? oversize : len;
+      oversize_used = LWIP_MIN(space, LWIP_MIN(oversize, len));
       pos += oversize_used;
       oversize -= oversize_used;
       space -= oversize_used;