diff --git a/src/filo-fs-ext2.adb b/src/filo-fs-ext2.adb
index 7cf6147..21cfe0c 100644
--- a/src/filo-fs-ext2.adb
+++ b/src/filo-fs-ext2.adb
@@ -56,7 +56,7 @@
       end if;
 
       State.Part_Len := Part_Len;
-      State.First_Data_Block := Block_Offset (Read_LE32 (Super_Block, 5 * 4));
+      State.First_Data_Block := FSBlock_Offset (Read_LE32 (Super_Block, 5 * 4));
 
       declare
          S_Log_Block_Size : constant Unsigned_32 := Read_LE32 (Super_Block, 6 * 4);
@@ -110,8 +110,9 @@
             declare
                S_Desc_Size : constant Unsigned_16 := Read_LE16 (Super_Block, 63 * 4 + 2);
             begin
-               if S_Desc_Size in
-                  Unsigned_16 (Desc_Size'First) .. Unsigned_16 (Desc_Size'Last)
+               if Is_Power_Of_2 (S_Desc_Size) and then
+                  S_Desc_Size in Unsigned_16 (Desc_Size'First) ..
+                  Unsigned_16 (Positive'Min (Desc_Size'Last, 2 ** State.Block_Size_Bits))
                then
                   State.Desc_Size := Desc_Size (S_Desc_Size);
                else
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;