Cannot access index field, why?

I have a simple problem (or so I think) which kept me up all night:

 - Two Paradox tables.
 - One contains a reference to the other table. At this time, only
   some records actually does this, while most only have a blank entry (null).
 - Both have a primary key
 - I can select the index in the ttable component, but it doesn't seem to
   rearrange the attached grid control. In fact, it behaves like the
   primary index. Selecting the name-index works fine.
 - The end result is that I can link them together without Delphi complaining,
   until I reopen the other TTable or actually try to run the thing...

I need a list of all the companies within a category...

Using a DBLookupCombo will allow me to select a category/trade for the
selected company, but the other way around gives me that same error message:
"Cannot access index field 'tradeID'."

I've recreated all the indexes using DBD. That is, I erased them and then
redefined them. No change.

...help?

TIA

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