With newer versions of ImageJ/FIJI the ability of DiameterJ to analyze a single image is broken. I have moved on from the project so I have to do updates in my free time these days and over the last year I’ve been too busy to make any updates. Apologies. However, I do believe if you select yes to analyzing multiple images and have at least two images in a directory that you’d like to analyze the above error won’t occur. As a work around if you have two images, just copy and paste the same image in the directory twice.
I’m sorry for the inconvenience, as soon as I get the time I’ll update the code. Right now its written in the macro language of ImageJ. This summer I intend to update to native JAVA and that should help build more robustness into tool. Again apologies for the work around and let me know if the above doesn’t solve the problem.
Also, as an unrelated aside, I HEAVILY recommend everyone use FIJI for image analysis rather than ImageJ as ImageJ is no longer in active development.