[Exim] Setting spam threshold via LDAP using Exiscan

I'm interseted in using the exiscan patch with Exim 4.12.

One issue I have is that I would like to specify the spam threshold using
an LDAP lookup. There is a configuration option for exiscan called
'exiscan_spamd_threshold'.

Using something like this:
exiscan_spamd_threshold = ${lookup ldap {...}}

returns a value that returns the following error from exim:
"fixed-point number expected for exiscan_spamd_treshold"

Obviously exiscan_spamd_threshold expects a number. Am I going down the
wrong path, or is this possible?

thanks
Paul