diff --git a/fold.c b/fold.c
index 1d88ed9..8019125 100644
--- a/fold.c
+++ b/fold.c
@@ -85,7 +85,8 @@ main(int argc, char *argv[])
 		width = estrtonum(EARGF(usage()), 1, MIN(LLONG_MAX, SIZE_MAX));
 		break;
 	ARGNUM:
-		width = estrtonum(argv[0], 1, MIN(LLONG_MAX, SIZE_MAX));
+		if (!(width = ARGNUMF()))
+			eprintf("illegal width value, too small\n");
 		break;
 	default:
 		usage();