Why is my list total showing as "Count= undefined"?

I'm updating a view on an existing list programmatically but I can't get the aggregation to work correctly. I'm setting it as follows (the values are pulled from an xml site definition):

if (AggregationStatus == "On")
  {
    view.Aggregations = "<FieldRef Name=\"" + AggViewField.Name + "\" Type=\"" +      AggViewField.Type + "\"/>";
    view.AggregationsStatus = AggregationStatus;
  }
  ... more settings here ...
    view.Update();

And if I look at the list with SharePoint Manager I can see the aggregation has been applied as follows:

 <Aggregations Value="On">
      <FieldRef Name="LinkFilename" Type="Count" />
 </Aggregations>

Which looks correct to me, but on the site itself the aggregation shows as "Count= undefined". If I modify the view I can see that the Count total is set for the name column and if I now save the view without changing anything the aggregation works properly, which makes me wonder if the view.Update() isn't doing what I think it is. Do I need to update the list as well? Is there some kind of SP_voodoo required to get an aggregation to work?

Any suggestions gratefully accepted.

This is SharePoint 2013 Enterprise on premises and my solution is using VS 2012