Not enough data to calculate spout lag

[Sachin Pasalkar]

Hi,

I found log "Metrics Tick: Not enough data to calculate spout lag.” in my 
topology and then topology becomes inactive.

I check the source:
  if (_partitions != null && _partitions.size() == _partitionToOffset.size()) {
……}else {
   LOG.info("Metrics Tick: Not enough data to calculate spout lag.");
}

What situation will cause  _partitions != null or _partitions.size() 
==_partitionToOffset.size()?

Thanks,
Sachin