Current bitcoin-master on testnet, `maxconnections=8` in the config file. Running in VM that doesn't have public IP and is not reachable from the Internet.

Wait until you have 8 peers.

```
telnet localhost 18333
```

It will result in:

```
Assertion failed: (nMaxInbound > 0), function AcceptConnection, file net.cpp, line 973.
```

First found in https://github.com/bitcoin/bitcoin/pull/8996, but can be reproduced on master.

CBanEntry banEntry(GetTime());
    banEntry.banReason = banReason;
    if (bantimeoffset <= 0)
    {
        bantimeoffset = GetArg("-bantime", DEFAULT_MISBEHAVING_BANTIME);
        sinceUnixEpoch = false;
    }
    banEntry.nBanUntil = (sinceUnixEpoch ? 0 : GetTime() )+bantimeoffset;
    {
        LOCK(cs_setBanned);
        if (setBanned[subNet].nBanUntil < banEntry.nBanUntil) {
            setBanned[subNet] = banEntry;
            setBannedIsDirty = true;
        }
        else
            return;
    }
    if(clientInterface)
        clientInterface->BannedListChanged();
    {
        LOCK(cs_vNodes);
        BOOST_FOREACH(CNode* pnode, vNodes) {
            if (subNet.Match((CNetAddr)pnode->addr))
                pnode->fDisconnect = true;
        }
    }
    if(banReason == BanReasonManuallyAdded)
        DumpBanlist(); 
}
bool CConnman::Unban(const CNetAddr &addr) {