diff --git a/src/fs-filo.ads b/src/fs-filo.ads
index d8b5998..a3d8f31 100644
--- a/src/fs-filo.ads
+++ b/src/fs-filo.ads
@@ -6,6 +6,7 @@
 package FS.FILO is
 
    type Partition_Length is range 0 .. Interfaces.Unsigned_64'Last;
+   subtype Partition_Offset is Partition_Length range 0 .. Partition_Length'Last - 1;
 
    type File_Length is range 0 .. int'Last; -- Should be higher, fix FILO first
    subtype File_Offset is File_Length;