I have built a chassis using structural members and done this 100's of times, but I am getting this non-linear edge invalid error on a linear edge when trying to insert bends?? What am I missing?

I have attached the part, I have used this procedure with success before and never encountered this error. I have checked that the edge is in fact a straight line so I am at a loss as to what the problem is. The purpose of doing this is to create templates for the laser cutter to cope the tubes.

Any help appreciated, thanks.