Search fails after adding index

Hi,

I am trying to index searching on uid. However as soon as I add the
following index to slapd.conf:

index           uid pres,eq,sub

stop slapd, run 'su openldap -c slapindex' and start slapd again, searching
for any uid returns 0 results.

I am using the following search command:

ldapsearch -x "(&(objectClass=posixAccount)(uid=laurens))"

After removing the index line from slapd.conf and restarting slapd the
search returns the desired result again. I set the loglevel to 480, but the
log does not contain any entries which indicate a problem.

Why does adding indexes result in the search failing?

I am running OpenLDAP 2.4.17 on Debian testing with the hdb backend.

TIA,

Laurens