diff --git a/sys/posix/pthread/pthread.c b/sys/posix/pthread/pthread.c
index cbd237c6328f..eeb553e219a8 100644
--- a/sys/posix/pthread/pthread.c
+++ b/sys/posix/pthread/pthread.c
@@ -87,7 +87,7 @@ static void *pthread_start_routine(void *pt_)
 
 static int insert(pthread_thread_t *pt)
 {
-    int result = -1;
+    int result = KERNEL_PID_UNDEF;
     mutex_lock(&pthread_mutex);
 
     for (int i = 0; i < MAXTHREADS; i++){
@@ -160,7 +160,7 @@ int pthread_create(pthread_t *newthread, const pthread_attr_t *attr, void *(*sta
                                    pthread_start_routine,
                                    pt,
                                    "pthread");
-    if (pt->thread_pid == KERNEL_PID_UNDEF) {
+    if (!pid_is_valid(pt->thread_pid)) {
         free(pt->stack);
         free(pt);
         pthread_sched_threads[pthread_pid-1] = NULL;