a error ? yes, maybe (with xx)
but certainly not on line 11

with "bts ebx,1" or "bts ebx,3" ...
it does compile without problem.

but with "bts ebx,2, it doesn't compile.

Take a look at http://board.flatassembler.net/topic.php?t=12790

If you replace all "xx" occurrences with "2" it becomes more obvious the relationship with the link above.

ok, understood (tks Loco)
indeed, this below does compile

If the preprocessor is supposedly correct in the treatment of numbers then a fix could be modifying PROC to make it declare an assembly-time variable with the name of the parameter suffixed with some local symbol. I think that should be enough to restrict the valid names to the ones the assembler stage accepts as name symbols.