Under 0.11.2 I never had a problem creating a 0-tx fee. But after upgrading to 0.12, I always get the error "mempool min fee not set" when I try to send it. There's an ugly workaround of calling prioritisetransaction as described here:
https://git.io/vabUJ. But that doesn't work if the mempool is already full, so I first have to restart the daemon to clear the mempool, before I can send the reprioritised tx.Is there a workaround that doesn't require restarting the daemon?
With Bitcoin Core 0.12, the wallet will never create transactions less than the minimum relay fee. That value has a default which I think is 0.00005. You can use
-mintxfee=0 flag in the startup command to set the minimum relay fee to 0 but that will also have the effect of your node also relaying transactions with no fees.
Also, why are you trying to create free transactions? You know that they will probably not get confirmed.