Invalid char set error when trying to batch convert files

[Zentian]

I'm trying to batch convert some images and am getting an invalid char set error when I do. I'm on Windows 10 and I'm using.

myconverter.scr in %AppData%/Roaming/Gimp/2.10/scripts/

Code:

(define (myconvert in_filename out_filename)
    (let* (
            (image (car (gimp-file-load RUN-NONINTERACTIVE in_filename in_filename)))
            (drawable (car (gimp-image-get-active-layer image)))
        )
        (file-heif-save RUN-NONINTERACTIVE image drawable out_filename out_filename 0 0)
    )
)

then a .bat file

Code:

 for /r %%i in (*.jpg) ; do "C:\Program Files\GIMP 2\bin\gimp-2.10.exe" -i -b '(myconvert "%%i" "%%i.HEIF")' -b '(gimp-quit 0)' ; done

It looks like it's able to open the image, but then prints out 

** (gimp-2.10.exe:21188): WARNING **: 19:22:33.507: Invalid charset: `InvalidCharsetId'
batch command executed successfully
batch command executed successfully

Does anyone know what the issue could be?