diff --git a/ArcFormats/ArcMRG.cs b/ArcFormats/ArcMRG.cs
index 5f60bb3b..f99d8ea8 100644
--- a/ArcFormats/ArcMRG.cs
+++ b/ArcFormats/ArcMRG.cs
@@ -160,7 +160,7 @@ namespace GameRes.Formats.FC01
             if (last_offset != actual_offset)
                 return null;
 
-            while (++remaining < index.Length)
+            while (remaining++ < index.Length)
                 key -= (byte)remaining;
 
             return key;