Hi Tom, also what radius value are you hoping to use there? It appears that some larger values do end up working with your existing geometry, like for example radius = 0.5 seems to work ok over here (there is one edge missing in your yellow edges, make sure it's also selected before doing the fillet).

To get the fillets to perform better would probably involve smoothing out that creased edge which will probably involve some trimming and blending work, let me know if you want me to do that.

- Michael