diff --git a/src/v1.rs b/src/v1.rs
index 3386644..72b4257 100644
--- a/src/v1.rs
+++ b/src/v1.rs
@@ -285,7 +285,7 @@ impl std::error::Error for Error{}
 #[binrw]
 #[brw(little)]
 #[derive(Debug,Clone,Copy)]
-pub struct BlockId(u32);
+pub struct BlockId(#[br(map=|i:u32|i-1)]u32);
 
 #[binrw]
 #[brw(little)]