See images. I think this is the second time this has happened on this playthrough. The rest of the time, I get one or maybe two slave coffins, as described in this already-solved-in-1.9B2 bug. I don't know if the two bugs are related.
The small number of coffins is solved in the (not yet released) Beta 2. Still broken in Beta 1
However the other bug has not been reported yet. Surely have not had it in any of my playtroughs.
Perhaps you can also attach your savegame file and debuglog to your post?
Flew to a different slave camp with the same message (must be the other one I'm remembering) and attached files.
I can confirm this is the same bug. If (due to the bug) there are 0 coffins, then you get the angry response.
Although this won't happen again in 1.9 Beta 2, it will not retroactively fix this save file.