No eq index on uniquemember allowed?

[Adam Tauno Williams <adam@morrison-ind.com>]

If I try

index   uniquemember,roleoccupant                       eq,pres

with slapindex I get a 

equality index of attribute "uniquemember" disallowed

But it would seem I REALLY need an index on this attribute.  So how does one go
about creating it?

Systems and Network Administrator
Morrison Industries
1825 Monroe Ave NW.
Grand Rapids, MI. 49505