This is issue #198. If you end up with undefs in d.data, but d.na is all true's, indexing into that should return NA. That's the DataArray you get from using `a = DataArray(Array(ASCIIString, 5))`. We need one of two rules:

1. If at all possible, don't allow undefs in d.data.

2. Before indexing into d.data, check d.na to see if there are NAs. 

As part of a commit to start to address #198, I used option 2, but option 1 may be the better approach.