Invalid expression for .BASE directive.

    Error: The expression is not valid for .BASE directive.

    User Action: The expression you specified for a base register
    with the .BASE directive should contain no forward references and
    resolve to one of the following at this point in assembly: psect
    +/- offset, external symbol reference +/- offset, integer, label
    +/- offset, where the label is defined in a psect with the EXE
    and NOMIX attributes. See Chapter 5 in the Reference Manual for
    more information about the assembler evaluation of expressions.