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.
{
    if (!fCheckBlockIndex) {
        return;
    }
    LOCK(cs_main);
    
    
    
    if (chainActive.Height() < 0) {
        assert(mapBlockIndex.size() <= 1);
        return;
    }
    
    std::multimap<CBlockIndex*,CBlockIndex*> forward;
    for (auto& entry : mapBlockIndex) {
        forward.insert(std::make_pair(entry.second->pprev, entry.second));
    }
    assert(forward.size() == mapBlockIndex.size());
    std::pair<std::multimap<CBlockIndex*,CBlockIndex*>::iterator,std::multimap<CBlockIndex*,CBlockIndex*>::iterator> rangeGenesis = forward.equal_range(nullptr);
    CBlockIndex *pindex = rangeGenesis.first->second;
    rangeGenesis.first++;
    assert(rangeGenesis.first == rangeGenesis.second);