servfail for stub-zones

[A. Schulze via Unbound-users]

Hello,

an unbound instance have this configuration to answer an dnsbl with  
data from a local rbldnsd:

    server:
     domain-insecure: "zen.spamhaus.org."
    stub-zone:
     name: "zen.spamhaus.org."
     stub-addr: 192.0.2.1
     stub-addr: 192.0.2.2

Also, I've "log-replies: yes"

I do expect logs with NOERROR or NXDOMAIN reply_codes. But I also see  
0.01% SERVFAIL.

That's what I do not understand. What could be a reason for unbound's  
answer "SERVFAIL"?

The only reason I'm aware /could/ be the fact, that rbldnsd never  
answer via TCP.
But as far as I know, I can't tell unbound "this stub servers are  
reachable via UDP only"

Andreas