Coordinator selected invalid assignment protocol: null

PartitionAssignor assignor = lookupAssignor(assignmentStrategy);
    if (assignor == null)
        throw new IllegalStateException("Coordinator selected invalid
assignment protocol: " + assignmentStrategy);

private PartitionAssignor lookupAssignor(String name) {
    for (PartitionAssignor assignor : this.assignors) {
        if (assignor.name().equals(name))
            return assignor;
    }
    return null;
}