I had the same problem with the LensID information. My camera includes no such data in it's exif information. Yours probably omits the ISO (maybe?). You can test it by running
(using your image filename, of course), and looking at what fields are provided.
I think the script could use some exception handling improvement when the asked-for data is missing.
At the risk of putting words into somebody else's mouth, it probably is true that the script is more of a proof of concept, and was envisioned as needing to be customized for your own caption preferences.
Even if you are not a programmer, you might play with the script, by (eg) commenting out (insert a '#') the line looking for ISO.