FB v.1.5.3.4870

I've been evolving this db for a few years.
Now I need to use UNICODE for Private Use Area characters.

For the create script, changing only
DEFAULT CHARACTER SET WIN1252
to
DEFAULT CHARACTER SET UNICODE_FSS

is returning the error:

ISC ERROR CODE:335544351
unsuccessful metadata update
key size too big for index TYP1LAST1FIRST1IDX

The index is based upon three fields:
TYP1 BIGINT
FirstString1 VARCHAR(36)
LastString1 VARCHAR(84)

The actual PK is still INTEGER.
I need TYP1 BIGINT to store the numeric equivalent
of an enumerated type.

This TYP1 and the two VARCHAR fields are the essential
organizing data for my application.

Any suggestions will be greatly appreciated.