diff --git a/src/filo-fs-ext2.ads b/src/filo-fs-ext2.ads
index 13e23f8..63f817e 100644
--- a/src/filo-fs-ext2.ads
+++ b/src/filo-fs-ext2.ads
@@ -66,12 +66,12 @@
    subtype Inode_Extents_Index is Natural range 0 .. 59;
 
    subtype Inode_Size is Positive range 128 .. Positive (Unsigned_16'Last);
-   subtype Desc_Size  is Positive range  32 .. Positive (Unsigned_16'Last);
+   subtype Desc_Size  is Positive range  32 .. 2 ** 15; -- power-of-2 that fits in 16 bits
 
    type T is record
       S : State;
       Part_Len          : Partition_Length := 0;
-      First_Data_Block  : Block_Offset := 0;
+      First_Data_Block  : FSBlock_Offset := 0;
       Block_Size_Bits   : Log_Block_Size := 10;
       Inodes_Per_Group  : Positive := 1;
       Inode_Size        : Ext2.Inode_Size := Ext2.Inode_Size'First;