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 
return NullUniValue;
}
static UniValue invalidateblock(const JSONRPCRequest& request)
{
    if (request.fHelp || request.params.size() != 1)
        throw std::runtime_error(
            RPCHelpMan{"invalidateblock",
                "\nPermanently marks a block as invalid, as if it violated a consensus rule.\n",
                {
                    {"blockhash", RPCArg::Type::STR_HEX, RPCArg::Optional::NO, "the hash of the block to mark as invalid"},
                },
                RPCResults{},
                RPCExamples{
                    HelpExampleCli("invalidateblock", "\"blockhash\"")
            + HelpExampleRpc("invalidateblock", "\"blockhash\"")
                },
            }.ToString());
    uint256 hash(ParseHashV(request.params[0], "blockhash"));
    CValidationState state;
    {
        LOCK(cs_main);
        CBlockIndex* pblockindex = LookupBlockIndex(hash);
        if (!pblockindex) {
            throw JSONRPCError(RPC_INVALID_ADDRESS_OR_KEY, "Block not found");
        }
        InvalidateBlock(state, Params(), pblockindex);
    }
    if (state.IsValid()) {
        ActivateBestChain(state, Params());
    }