Do not return the same value for no encryption and error.
diff --git a/src/ziptool.c b/src/ziptool.c
index 7e1302e..2994cb2 100644
--- a/src/ziptool.c
+++ b/src/ziptool.c
@@ -667,7 +667,7 @@
         return ZIP_EM_AES_256;
     else if (strcmp(arg, "unknown") == 0)
         return 100;
-    return 0; /* TODO: error handling */
+    return (zip_uint16_t)-1; /* TODO: error handling */
 }
 
 static void