i just tested this with version 2.7.9 as offline version and i get the same error, that is weird and i think that was a bug in that version, and it is fixed when you upgrade to a newer version as i said above.
also
I then booted into Xubuntu 16.04 Live CD and
installed latest electrum,
restored wallet from seed,
{here you download the blockchain headers and sync since you are not yet disconnected!}
disconnected the PC and
signed my transaction no problem, then powered off the PC.