Reported here https://github.com/bitcoin/bitcoin/pull/12629#issuecomment-381095603 by @Empact.  I've seen it now in a few different travis jobs, including:

https://travis-ci.org/bitcoin/bitcoin/jobs/366157920
https://travis-ci.org/bitcoin/bitcoin/jobs/366139851

From my initial investigation it seems like there may be an actual bug in block announcement, so I'm  opening this issue to track.
{
    AssertLockHeld(cs_main);
    AssertLockHeld(mempool.cs);
    CBlockIndex* tip = chainActive.Tip();
    assert(tip != nullptr);
    
    CBlockIndex index;
    index.pprev = tip;
    
    
    
    
    
    
    index.nHeight = tip->nHeight + 1;
    std::pair<int, int64_t> lockPair;
    if (useExistingLockPoints) {
        assert(lp);
        lockPair.first = lp->height;
        lockPair.second = lp->time;
    }
    else {
        
        CCoinsViewMemPool viewMemPool(pcoinsTip.get(), mempool);
        std::vector<int> prevheights;
        prevheights.resize(tx.vin.size());
        for (size_t txinIndex = 0; txinIndex < tx.vin.size(); txinIndex++) {