"Error 41 running diskmap, line 100: Bad arithmetic conversion" when USER DIRECT contains TDISK entries

[RossPatterson]

The DISKMAP EXEC command on the MAINT 5E5 disk generates error message Error 41 running diskmap, line 100: Bad arithmetic conversion and aborts when processing a DIRECT that contains TDISK entries. For example:

USER TEST TEST 16M 16M G
...
MDISK   191 3350 T-DISK 10