Prefetch not working?

[Mitch Claborn]

I am running ubound 1.5.4 on pfSense 2.2.6.
The /var/unbound/unbound.conf file contains

...
prefetch: yes
prefetch-key: no
....

Yet when I show the statistics, it seems like nothing is getting 
pre-fetched:

total.num.queries=13503
total.num.cachehits=6383
total.num.cachemiss=7120
total.num.prefetch=0

Is there something else I need to do to get the prefetch working?

-- 

Mitch