DKIM passes everywhere apart from Yahoo!

I'm using dkim-milter, Postfix on Ubuntu (I think I used these instructions for setting up). Anyway, using the reflectors such as Port25, BlackOps and Altn.com I get passes for DKIM:

X-DKIM: OpenDKIM Filter v2.0.1 medusa.blackops.org o2SGTMSg005616
Authentication-Results: medusa.blackops.org; dkim=pass (1024-bit key)
       header.i=@example.com; dkim-adsp=pass

dkim=pass header.d=example.com (b=miSIxi7TMX; 1:0:good);

Authentication-Results: verifier.port25.com header.d=example.com; dkim=pass (matches From: ian@example.com);

Yahoo gives this:

Authentication-Results: mta1031.mail.ukl.yahoo.com  from=; domainkeys=neutral (no sig);  from=example.com; dkim=permerror (key failed)

Where, obviously, example.com is my site address. Is anyone aware of anything different with Yahoo! that would stop these from signing?

—
As an update, Yahoo! has given me a pass as well, but only once, and then I sent an email again, and it failed. dk.elandsys.com has given me a fail and said:

Authentication-Results: ns1.qubic.net; dkim=permerror
       (verification error: s=mail d=example.com: d2i_PUBKEY_bio() failed)
       header.i=@example.com; dkim-adsp=unknown

I couldn't find anything useful though on t'internet about this message.

TIA