diff --git a/kernel/system/do_schedctl.c b/kernel/system/do_schedctl.c
index 592b7d4f2..a4ae1655f 100644
--- a/kernel/system/do_schedctl.c
+++ b/kernel/system/do_schedctl.c
@@ -9,12 +9,16 @@
 PUBLIC int do_schedctl(struct proc * caller, message * m_ptr)
 {
 	struct proc *p;
+	int proc_nr;
 
 	/* Only system processes can change process schedulers */
 	if (! (priv(caller)->s_flags & SYS_PROC))
 		return(EPERM);
 
-	p = proc_addr(_ENDPOINT_P(m_ptr->SCHEDULING_ENDPOINT));
+	if (!isokendpt(m_ptr->SCHEDULING_ENDPOINT, &proc_nr))
+		return EINVAL;
+
+	p = proc_addr(proc_nr);
 	p->p_scheduler = caller;
 
 	return(OK);
diff --git a/kernel/system/do_schedule.c b/kernel/system/do_schedule.c
index 044707673..bf68c43da 100644
--- a/kernel/system/do_schedule.c
+++ b/kernel/system/do_schedule.c
@@ -10,6 +10,10 @@
 PUBLIC int do_schedule(struct proc * caller, message * m_ptr)
 {
 	struct proc *p;
+	int proc_nr;
+
+	if (!isokendpt(m_ptr->SCHEDULING_ENDPOINT, &proc_nr))
+		return EINVAL;
 
 	p = proc_addr(_ENDPOINT_P(m_ptr->SCHEDULING_ENDPOINT));