diff --git a/contrib/ports/unix/port/sys_arch.c b/contrib/ports/unix/port/sys_arch.c
index fa7ea72fb..a5fd18b33 100644
--- a/contrib/ports/unix/port/sys_arch.c
+++ b/contrib/ports/unix/port/sys_arch.c
@@ -690,7 +690,7 @@ sys_thread_sem_alloc(void)
   err_t err;
   int ret;
 
-  sem = (sys_sem_t*)malloc(sizeof(sys_sem_t*));
+  sem = (sys_sem_t*)malloc(sizeof(sys_sem_t));
   LWIP_ASSERT("failed to allocate memory for TLS semaphore", sem != NULL);
   err = sys_sem_new(sem, 0);
   LWIP_ASSERT("failed to initialise TLS semaphore", err == ERR_OK);