The error seems to relate to this code.

Code:
 HChkSum = $E4 + $06 + $02 + ImaMem + RL        'Calculate Read Chksum
HChkSum = $100 - HChkSum                    'Calculate two's complement
HChkSum is a byte variable and I assume must be overflowing as the additions take place or when we try to subtract 100H from it.