bug described by Val is caused by missing experimental system data which we're now using from biogrid but don't have for pombase. Fix is just to always supply a blank experimental system (empty string) for pombase data.
original method to reproduce: If you go from cdc2 interaction page. Select cut3, add interactions from BioGRID (interactions are retrieved and can be imported) Then change to add physical interactions from PomBase (interactions can be retrieved. The error occurs when you try to display these interactions).