Cannot specify value for "EXIF focal length in 35mm Format"

I have a 9mm (in APS-C) lens. In the preset manager, I can set the “EXIF focal length” to 9, but when I try to set the “EXIF focal length in 35mm format” using “Edit current value,” it just give me an error sound and does not set the value. I also tried using “find and replace text” but it never succeeds. How to make this work???