diff --git a/src/core/tcp_out.c b/src/core/tcp_out.c
index 8ccdffb9..4d39826c 100644
--- a/src/core/tcp_out.c
+++ b/src/core/tcp_out.c
@@ -467,7 +467,7 @@ tcp_write(struct tcp_pcb *pcb, const void *arg, u16_t len, u8_t apiflags)
 #endif /* TCP_OVERSIZE_DBGCHECK */
     oversize = pcb->unsent_oversize;
     if (oversize > 0) {
-      LWIP_ASSERT("inconsistent oversize vs. space", oversize_used <= space);
+      LWIP_ASSERT("inconsistent oversize vs. space", oversize <= space);
       seg = last_unsent;
       oversize_used = LWIP_MIN(space, LWIP_MIN(oversize, len));
       pos += oversize_used;