[Exim] setting SA variable from exim

Hi, not sure if this is an exiscan or exim problem, but I'm trying to set a variable for exiscan which is resulting in an error:

The code:

exiscan_spamd_condition = ${if eq \
    {${mask:$host_address/16}}\
    {${mask:$sender_host_address/16}}\
    {0} {1}\
}

(which seems to correspond with the documentation)

The error:

exiscan: warning - failure while expanding exiscan_spamd_condition: "" is not an IP address

now this works for:

exiscan_spamd_condition = ${if eq \
    {$host_address/16}\
    {$sender_host_address/16}\
    {0} {1}\
}

(but of course without the desired functionality!)

Any ideas would be a great help,

Alexander Smith