Rather than deal with nulls, I did a selection [Problem Field] IS NULL then used the field calculator to replace <null> with ""
Not very elegant, a bit tedious, but it works and then allows my old concatenation calculation to work properly. I've wasted enough time on solving this.
Dan, in another post you talk about not having tables with nulls. How do you create a new feature class in a geodatabase that contains no nulls?
Dave