diff --git a/patterns/mp4.hexpat b/patterns/mp4.hexpat
index 45cac94..2834dfb 100644
--- a/patterns/mp4.hexpat
+++ b/patterns/mp4.hexpat
@@ -424,7 +424,7 @@ struct MovieBox : BaseBox {
 };
 
 struct MediaDataBox : BaseBox {
-    u8 data[while($ < endOffset)] [[sealed]];
+    std::mem::Bytes<boxSize - sizeof(size) - sizeof(type)> data;
 };
 
 struct Box {