Mining the Litecoin testnet with minerd

I have set up the Litecoin testnet locally on my computer, two nodes running connected to each other and no one else. I’ve downloaded CPUMiner 2.5.0 (minerd) and run the following.

minerd -a scrypt -o http://127.0.0.1:19332 -u username -p Ghd8s7Sw13Yd -t 1 -D --no-stratum --no-longpoll --no-getwork --coinbase-addr=myp43yPrRwodbzeB8ChfqT7d53zYqdK71R

However I get the following error.

[2017-08-03 09:13:50] 1 miner threads started, using 'scrypt' algorithm.
[2017-08-03 09:13:50] HTTP request failed: The requested URL returned error: 500 Internal Server Error
[2017-08-03 09:13:50] json_rpc_call failed, retry after 30 seconds

Contents of my conf file for the client I’m trying to mine against. 19332 is the default testnet RPC port so does not need to be specified. The other local client has a different P2P and RPC port so they do not clash but I’m not able to mine against that one either.

rpcuser=username
rpcpassword=Ghd8s7Sw13Yd
rpcallowip=127.0.0.1
rpcallowip=192.168.1.155
server=1
listen=1
daemon=1

# Run on the test network instead of the real bitcoin network.
testnet=1

# Run a regression test network
#regtest=0

Any ideas what I need to do to be able to CPU mine the testnet?