Works fine for me without an error. Are you using the app store version of Finereader or the direct download?
I should have specified but this script works with the direct download version. The applescript of the direct download version is significantly different due to Sandboxing. I have not tried the app store version. You might try contracting Abby directly. It seems they are pretty helpful in resolving differences with applescript between the two versions.
Frederiko