diff --git a/src/core/timeouts.c b/src/core/timeouts.c
index de5300ad..4196f9b4 100644
--- a/src/core/timeouts.c
+++ b/src/core/timeouts.c
@@ -147,6 +147,8 @@ tcpip_tcp_timer(void *arg)
 void
 tcp_timer_needed(void)
 {
+  LWIP_ASSERT_CORE_LOCKED();
+
   /* timer is off but needed again? */
   if (!tcpip_tcp_timer_active && (tcp_active_pcbs || tcp_tw_pcbs)) {
     /* enable and start timer */