diff --git a/src/filo-blockdev.adb b/src/filo-blockdev.adb
index 28747e7..ba7bc6d 100644
--- a/src/filo-blockdev.adb
+++ b/src/filo-blockdev.adb
@@ -2,7 +2,7 @@
 
    procedure Read
       (Buffer  : in out Buffer_Type;
-       Offset  : in     Natural;
+       Offset  : in     Blockdev_Offset;
        Success :    out Boolean)
    is
       use Interfaces.C;
diff --git a/src/filo-blockdev.ads b/src/filo-blockdev.ads
index d35eef3..2fde36a 100644
--- a/src/filo-blockdev.ads
+++ b/src/filo-blockdev.ads
@@ -5,8 +5,10 @@
 
    Procedure Read
       (Buffer  : in out Buffer_Type;
-       Offset  : in     Natural;
-       Success :    out Boolean);
+       Offset  : in     Blockdev_Offset;
+       Success :    out Boolean)
+   with
+      Pre => Offset <= Blockdev_Length'Last - Buffer'Length;
 
 private
 
diff --git a/src/filo.ads b/src/filo.ads
index c0a7e3d..93985b1 100644
--- a/src/filo.ads
+++ b/src/filo.ads
@@ -4,8 +4,10 @@
 
 package 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 Blockdev_Length is range 0 .. Interfaces.Unsigned_64'Last;
+   subtype Blockdev_Offset is Blockdev_Length range 0 .. Blockdev_Length'Last - 1;
+   subtype Partition_Length is Blockdev_Length;
+   subtype Partition_Offset is Blockdev_Offset;
 
    BLOCK_SIZE : constant := 512;
    type Block_Offset is range 0 .. Partition_Offset'Last / BLOCK_SIZE;