In IndexCatalogImpl::_isSpecOk(), we use NamespaceString::isOplog() to see if the collection we're verifying the index spec for is the oplog collection. See here.

However, NamespaceString::isOplog() calls NamespaceString::oplog() which has the following check:

static bool oplog(StringData ns) {
    return ns.startsWith("local.oplog.");
}

The problem is that in IndexCatalogImpl::_isSpecOk(), our NamespaceString doesn't have the trailing "." that NamespaceString::oplog() checks for. Our NamespaceString would be "local.oplog".