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() != 0)
        throw std::runtime_error(
            RPCHelpMan{"getdifficulty",
                "\nReturns the proof-of-work difficulty as a multiple of the minimum difficulty.\n",
                {},
                RPCResult{
            "n.nnn       (numeric) the proof-of-work difficulty as a multiple of the minimum difficulty.\n"
                },
                RPCExamples{
                    HelpExampleCli("getdifficulty", "")
            + HelpExampleRpc("getdifficulty", "")
                },
            }.ToString());
    LOCK(cs_main);
    return GetDifficulty(chainActive.Tip());
}
static std::string EntryDescriptionString()