[exim] string expansion fail with missing } at end of string

Hello,

$ exim4 -be
> ${if match_ip{$sender_host_address}{ ${lookup dnsdb{>: defer_lax,a=${lookup dnsdb{>: defer_lax,mxh=$sender_address_domain}}}} } {no}{yes}}
2013-10-07 12:06:25 failed to expand " ${lookup dnsdb{>:
defer_lax,a=${lookup dnsdb{>: defer_lax,mxh=$sender_address_domain"
while checking a list: missing } at end of string

> ${if match_ip{$sender_host_address}{ : ${lookup dnsdb{mxh=$sender_address_domain}} }{yes}{no}}
2013-10-07 12:09:38 failed to expand " : ${lookup
dnsdb{mxh=$sender_address_domain" while checking a list: missing } at
end of string

while
> ${if match_ip{$sender_host_address}{ 1.2.3.4 : 5.6.7.8 }{yes}{no}}
no

> ${lookup dnsdb{mxh=$sender_address_domain}}
[empty answer, no error]


I fail to see why the first two fail expansion. I have replaced it
with a hosts= check but wondering what's going on.

Peter