diff --git a/usr.bin/w/w.c b/usr.bin/w/w.c
--- a/usr.bin/w/w.c
+++ b/usr.bin/w/w.c
@@ -400,7 +400,7 @@
 		ep->args = fmt_argv(kvm_getargv(kd, ep->kp, argwidth),
 		    ep->kp->ki_comm, NULL, MAXCOMLEN);
 		if (ep->args == NULL)
-			xo_err(1, NULL);
+			xo_err(1, "fmt_argv");
 	}
 	/* sort by idle time */
 	if (sortidle && ehead != NULL) {