diff --git a/sys/kern/kern_prot.c b/sys/kern/kern_prot.c
--- a/sys/kern/kern_prot.c
+++ b/sys/kern/kern_prot.c
@@ -2202,6 +2202,7 @@
 proc_set_cred_init(struct proc *p, struct ucred *newcred)
 {
 
+	MPASS(p->p_ucred == NULL);
 	p->p_ucred = crcowget(newcred);
 }