ksh does not report an error with unset position argument when -u option has been set

Issue

When -u option has been set in ksh, the shell does not report error for using unset position argument like $1 although it should:

-u
    When the shell tries to expand an unset parameter other than the '@' and '*' special parameters, it shall write a message to standard error and the expansion shall fail with the consequences specified in Consequences of Shell Errors.