diff --git a/src/filo-blockdev.adb b/src/filo-blockdev.adb
index 8658e05..5bb6732 100644
--- a/src/filo-blockdev.adb
+++ b/src/filo-blockdev.adb
@@ -19,7 +19,7 @@
    begin
       -- With 32-bit longs, the current C interface
       -- can't access more than 512 * 2 ** 32:
-      if Offset / BLOCK_SIZE > Blockdev_Length (unsigned_long'last) then
+      if Unsigned_64 (Offset / BLOCK_SIZE) > Unsigned_64 (unsigned_long'last) then
          Success := False;
          return;
       end if;
diff --git a/src/filo.ads b/src/filo.ads
index 6a9a4b8..6c21c8c 100644
--- a/src/filo.ads
+++ b/src/filo.ads
@@ -4,7 +4,7 @@
 
 package FILO is
 
-   type Blockdev_Length is range 0 .. Interfaces.Unsigned_64'Last;
+   type Blockdev_Length is range 0 .. Interfaces.Integer_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;