sshd: invalid public DH value

Has anybody seen these in their logs?

Dec DD HH:MM:SS web sshd[1979]: invalid public DH value: <= 1
Dec DD HH:MM:SS web sshd[1979]: Disconnecting: bad client public DH value

Any idea what they mean? We get lots of ssh probes, most of which can
be ignored, but I've never seen this sshd message before.
Could somebody be probing for a buffer overflow?

We're running, "OpenSSH_5.2p1, OpenSSL 0.9.8l 5 Nov 2009", on Linux,
kernel 2.6.24-26.