Reported here: https://github.com/bitcoin/bitcoin/pull/10244#issuecomment-378035305

Travis log here: https://travis-ci.org/bitcoin/bitcoin/jobs/361243719#L2810

Note the JSON exception:

```
test_framework.authproxy.JSONRPCException: The mempool was not loaded yet (-1) 
```

That exception was introduced here: https://github.com/bitcoin/bitcoin/commit/cb1e319fe9e198c9c5cf5236fe9af5a3d748b9e8#diff-a0c8f511d90e83aa9b5857e819ced344R1617

cc @jtimon 
{
    if (request.fHelp || request.params.size() != 1)
        throw std::runtime_error(
            RPCHelpMan{"preciousblock",
                "\nTreats a block as if it were received before others with the same work.\n"
                "\nA later preciousblock call can override the effect of an earlier one.\n"
                "\nThe effects of preciousblock are not retained across restarts.\n",
                {
                    {"blockhash", RPCArg::Type::STR_HEX, RPCArg::Optional::NO, "the hash of the block to mark as precious"},
                },
                RPCResults{},
                RPCExamples{
                    HelpExampleCli("preciousblock", "\"blockhash\"")
            + HelpExampleRpc("preciousblock", "\"blockhash\"")
                },
            }.ToString());
    uint256 hash(ParseHashV(request.params[0], "blockhash"));
    CBlockIndex* pblockindex;
    {
        LOCK(cs_main);
        pblockindex = LookupBlockIndex(hash);
        if (!pblockindex) {
            throw JSONRPCError(RPC_INVALID_ADDRESS_OR_KEY, "Block not found");
        }
    }
    CValidationState state;
    PreciousBlock(state, Params(), pblockindex);
    if (!state.IsValid()) {
        throw JSONRPCError(RPC_DATABASE_ERROR, FormatStateMessage(state));
    }