Zcash getbalance error code -28

Hi
I’ve installed zcash on ubuntu. I try to do sample operation for example getbalance. But server throw error and exit.

root@1d45a51f1530:/home/zcash# ./src/zcash-cli getbalance “t1apTvGVJKMjaK9UdazAxSabm6DMPhQdkqM”
error code: -28
error message:
Loading block index…
[3]+ Exit 1 ./src/zcashd > /dev/null 2> /dev/null
root@1d45a51f1530:/home/zcash#

This is my config file:
testnet=0
addnode=mainnet.z.cash
rpcuser=username
rpcpassword=password
gen=1

What is the reason?