diff --git a/contrib/one-true-awk/run.c b/contrib/one-true-awk/run.c
--- a/contrib/one-true-awk/run.c
+++ b/contrib/one-true-awk/run.c
@@ -912,6 +912,9 @@
 		case 'c':
 			flag = 'c';
 			break;
+		case '\0':
+			FATAL("missing printf conversion specifier");
+			break;
 		default:
 			WARNING("weird printf conversion %s", fmt);
 			flag = '?';