?C?IILDOPTR UNRESOLVED EXTERNAL

I have some code that is doing address arithmetic on a (far) pointer in a structure. When I try and link I get the following error(s):

*** ERROR L127: UNRESOLVED EXTERNAL SYMBOL
    SYMBOL:  ?C?IILDOPTR
    MODULE:  .\obj\pixel.obj (PIXEL)
*** ERROR L128: REFERENCE MADE TO UNRESOLVED EXTERNAL
    SYMBOL:  ?C?IILDOPTR
    MODULE:  .\obj\pixel.obj (PIXEL)

This would appear to be a compiler library routine. (I.E. not in my code). If I remove the address arithmetic line, things link fine. The code compiles, links, and works fine on 4 other platforms.

Do I need to add a different library to link against? I'm using a new project for the DS80C390 in 22bit contiguous mode in v6.22.

I'll extract a code example if needed, but I was hoping someone would recognize the link error...