diff --git a/otherlibs/threads/unix.ml b/otherlibs/threads/unix.ml
index 34d97d874..62a322118 100644
--- a/otherlibs/threads/unix.ml
+++ b/otherlibs/threads/unix.ml
@@ -259,7 +259,7 @@ external link : string -> string -> unit = "unix_link"
 
 module LargeFile =
   struct
-    external lseek : file_descr -> int64 -> seek_command -> int = "unix_lseek_64"
+    external lseek : file_descr -> int64 -> seek_command -> int64 = "unix_lseek_64"
     external truncate : string -> int64 -> unit = "unix_truncate_64"
     external ftruncate : file_descr -> int64 -> unit = "unix_ftruncate_64"
     type stats =
diff --git a/otherlibs/unix/unix.ml b/otherlibs/unix/unix.ml
index 5065a47ae..da9afcf7f 100644
--- a/otherlibs/unix/unix.ml
+++ b/otherlibs/unix/unix.ml
@@ -216,7 +216,7 @@ external link : string -> string -> unit = "unix_link"
 
 module LargeFile =
   struct
-    external lseek : file_descr -> int64 -> seek_command -> int = "unix_lseek_64"
+    external lseek : file_descr -> int64 -> seek_command -> int64 = "unix_lseek_64"
     external truncate : string -> int64 -> unit = "unix_truncate_64"
     external ftruncate : file_descr -> int64 -> unit = "unix_ftruncate_64"
     type stats =
diff --git a/otherlibs/unix/unix.mli b/otherlibs/unix/unix.mli
index f4060ea80..ba3095144 100644
--- a/otherlibs/unix/unix.mli
+++ b/otherlibs/unix/unix.mli
@@ -343,7 +343,7 @@ val fstat : file_descr -> stats
 
 module LargeFile :
   sig
-    val lseek : file_descr -> int64 -> seek_command -> int
+    val lseek : file_descr -> int64 -> seek_command -> int64
     val truncate : string -> int64 -> unit
     val ftruncate : file_descr -> int64 -> unit
     type stats =
diff --git a/otherlibs/unix/unixLabels.mli b/otherlibs/unix/unixLabels.mli
index 069a2f9a8..346575115 100644
--- a/otherlibs/unix/unixLabels.mli
+++ b/otherlibs/unix/unixLabels.mli
@@ -347,7 +347,7 @@ val fstat : file_descr -> stats
 
 module LargeFile :
   sig
-    val lseek : file_descr -> int64 -> mode:seek_command -> int
+    val lseek : file_descr -> int64 -> mode:seek_command -> int64
     val truncate : string -> len:int64 -> unit
     val ftruncate : file_descr -> len:int64 -> unit
     type stats = Unix.LargeFile.stats =