Hi,
this is a known missing feature, relevant discussion + link to the jira ticket is here.
Unfortunately, at the moment we do not have free hands to implement support for missing values (me myself will not be at CERN until March) but PRs are of course welcome – or comments to the jira ticket explaining why the feature should float up the to do list.
The simplest workaround is to substitute missing values with some telltale value.
Cheers,
Enrico