Bug: https://bugs.gentoo.org/924227
--- a/src/zdmain.c
+++ b/src/zdmain.c
@@ -53,7 +53,7 @@ char *c;
         break;
     default:
         fprintf( stderr, "Usage: %s {<password> | <key0> <key1> <key2>} <cryptedzipfile> <plainzipfile>\n", argv[0] );
-        return;
+        return 0;
     }
 
     zipdecrypt( argv[argc-2], argv[argc-1], key0, key1, key2 );