Unhandled exception for undefined constant value

[iafisher]

To replicate:

SET(R1, N)

The problem is, the above is preprocessed into SETLO(R1, N); SETHI(R1, N). When substitute_label is called for SETLO, it catches the error and prints it, but not when it is called for SETHI (otherwise, two error messages would be printed for the same error).