GetLegendGraphic with Y OFFSET -99 crash mapserver

Mapserver 6.4.0 compiled under Debian 7.

If you request a legend with a WMS GetLegendGraphic request for a layer with OFFSET 1 -99 in its STYLE section, mapserv will crash with this error in the kern.log:

mapserv[3440]: segfault at 40 ip 00007feb2f0f33db sp 00007fffa294cf00 error 4 in libstdc++.so.6.0.17[7feb2f091000+e8000]

Mapserv will not crash for a normal WMS GetMap request and it renders fine.

The x offset value can be anything positive.
A positive y offset value does not crash mapserv.

I can provide a sample mapfile under request.