Some columns causing errors on computed fields

On this page I’ve made a code-type computed field for Length that does not show up if that column is NULL for a particular row. For the adjacent column, Full Interview, I made a text-type computed field, but when the column is NULL it puts in a link that refers back to the page itself. I’d like this to be empty when the column is NULL, but if I try to make a code-type field using that column I get an “ERR-Check Console”. I’ve tried different columns, and some trigger the error and some don’t. I can’t figure out what accounts for the difference. Any ideas?

The page I need help with: [log in to see the link]