diff --git a/lib/Lib_VTK_IO.f90 b/lib/Lib_VTK_IO.f90
index 4c6dbab5f..62a838189 100644
--- a/lib/Lib_VTK_IO.f90
+++ b/lib/Lib_VTK_IO.f90
@@ -481,7 +481,7 @@ contains
   integer function Get_Unit(Free_Unit)
   !---------------------------------------------------------------------------------------------------------------------------------
   implicit none
-  integer, intent(OUT), optional:: Free_Unit !< Free logic unit.
+  integer(I4P), intent(OUT), optional:: Free_Unit !< Free logic unit.
   integer::                        n1        !< Counter.
   integer::                        ios       !< Inquiring flag.
   logical::                        lopen     !< Inquiring flag.