TDBLookupComboBox and Keyboard Input Problem

I'm using D3 and have this anomaly:

I have a TDBLookupComboBox which correctly displays the contents of a
dictionary table using the mouse.  All items are 3 letter initials.
There are many items beginning with the letter "K".  If the user types
KAM (which is the 10th "K" item) in rapid sequence, the lookup goes to
the KAM item correctly.  If the user types KAM and a 4th letter rapidly,
an "Operation not applicable" error message pops up.  If the user types
KAM slowly, the box goes to the first item starting with "K", then the
first "A" item, then the first "M" item, and so on, so there is no 4th
letter error.

Can anyone help with this error?  Thanks,

Bob Yee