Using SQLT_VNU as an in out parameter fails

[shamis]

Following code can be used to reproduce the problem

DECLARE
    var1 number := 0;
BEGIN
    var1 := :SQLT_VNU_INOUT_VAR1;
    var1 := var1 + 1;
end;

Error produced is ORA-01459: Ungültige Länge für variable Zeichenfolge

Workaround would be to use a seperate IN and OUT parameters as follwoing

DECLARE
    var1 number := 0;
BEGIN
    var1 := :SQLT_VNU_IN_VAR1;
    var1 := var1 + 1;
    :SQLT_VNU_OUT_VAR2 := var1;
END;