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 
"  \"time\" : ttt,          (numeric) The block time in seconds since epoch (Jan 1 1970 GMT)\n"
            "  \"mediantime\" : ttt,    (numeric) The median block time in seconds since epoch (Jan 1 1970 GMT)\n"
            "  \"nonce\" : n,           (numeric) The nonce\n"
            "  \"bits\" : \"1d00ffff\", (string) The bits\n"
            "  \"difficulty\" : x.xxx,  (numeric) The difficulty\n"
            "  \"chainwork\" : \"0000...1f3\"     (string) Expected number of hashes required to produce the current chain (in hex)\n"
            "  \"nTx\" : n,             (numeric) The number of transactions in the block.\n"
            "  \"previousblockhash\" : \"hash\",  (string) The hash of the previous block\n"
            "  \"nextblockhash\" : \"hash\",      (string) The hash of the next block\n"
            "}\n"
                    },
                    RPCResult{"for verbose=false",
            "\"data\"             (string) A string that is serialized, hex-encoded data for block 'hash'.\n"
                    },
                },
                RPCExamples{
                    HelpExampleCli("getblockheader", "\"00000000c937983704a73af28acdec37b049d214adbda81d7e2a3dd146f6ed09\"")
            + HelpExampleRpc("getblockheader", "\"00000000c937983704a73af28acdec37b049d214adbda81d7e2a3dd146f6ed09\"")
                },
            }.ToString());
    uint256 hash(ParseHashV(request.params[0], "hash"));
    bool fVerbose = true;
    if (!request.params[1].isNull())
        fVerbose = request.params[1].get_bool();
    const CBlockIndex* pblockindex;
    const CBlockIndex* tip;
    {
        LOCK(cs_main);
        pblockindex = LookupBlockIndex(hash);
        tip = chainActive.Tip();