Index: contrib/one-true-awk/run.c
===================================================================
--- contrib/one-true-awk/run.c
+++ contrib/one-true-awk/run.c
@@ -912,6 +912,8 @@
 		case 'c':
 			flag = 'c';
 			break;
+		case '\0':
+			FATAL("missing printf conversion specifier");
 		default:
 			WARNING("weird printf conversion %s", fmt);
 			flag = '?';