diff --git a/src/filo-fs.adb b/src/filo-fs.adb
index ab2b9c8..69ed2ad 100644
--- a/src/filo-fs.adb
+++ b/src/filo-fs.adb
@@ -18,7 +18,10 @@
       Convention     => C,
       External_Name  => "filepos";
 
-   function Part_Len return Partition_Length is (Partition_Length (C_Part_Length));
+   function Part_Len return Partition_Length
+   is
+     (Partition_Length (unsigned_long'Min (
+        C_Part_Length, unsigned_long (Partition_Length'Last))));
 
    function File_Max return File_Length is
    begin