simnet generate block error

[leor-w]

Hi, I want to start a SIMNET to test my wallet service, but after I start the auto generate block option for a while, hcd throws an exception

command:
./hcd --simnet --rpcuser=leor --rpcpass=123456 --datadir="./blockchain_data" --rpclisten=0.0.0.0:10800 --generate --miningaddr=Ssi3tLCG8qo2rkofyQ9R6R9fxFvFeUxyxQX

error message:
[ERR] MINR: Unexpected error while processing block submitted via CPU miner: list size too small: 0 < 5

OS: Ubuntu 18.04