poudriere + bdb6 build issue (maybe newbie stupidity)

[Mike Andrews]

I’m experimenting with poudriere for the first time and running into an 
issue with some ports that use BDB.  With this in make.conf:

DEFAULT_VERSIONS= bdb=6

I get this for four ports that use BDB 6:

[00:00:47] ====>> [04][00:00:00] Finished build of databases/p5-BerkeleyDB: Ignored: cannot install: no eligible BerkeleyDB version. Requested: 6, incompatible: . Try: make debug-bdb
[00:00:47] ====>> [03][00:00:00] Finished build of databases/ruby-bdb: Ignored: cannot install: no eligible BerkeleyDB version. Requested: 6, incompatible: . Try: make debug-bdb
[00:00:48] ====>> [06][00:00:01] Finished build of www/webalizer: Ignored: cannot install: no eligible BerkeleyDB version. Requested: 6, incompatible: . Try: make debug-bdb
[00:00:49] ====>> [01][00:00:01] Finished build of textproc/redland: Ignored: cannot install: no eligible BerkeleyDB version. Requested: 6, incompatible: . Try: make debug-bdb

Installing the ports manually from source works just fine.

Am I missing something stupid/obvious here, or is it an issue with those 
specific four ports?