GIMP :: Load File As Layer In A Script

I can't get a simple script.scm to load a file, put it in a layer, add the layer to the active image and call it a day. My ultimate objective is to paste the same .png on top of every picture (about 70 of them) in batch mode. In order to feed the "load this particular file as a layer" command to Batch Image Manipulation plug-in, it has to be a script.

Despite a great deal of time on the internet looking for the proper syntax of gimp-load-layer, I could not find a single explanation to the "filename" parameter. Is it an absolute path? If relative, of what and where? If absolute, what's the proper syntax: do I put quotes, slash or backslash, one or two slash after C:? When I try to run my script, I get the error "Execution error for 'Test': Error: ( : 1) Invalid type for argument 2 to gimp-image-insert-layer, which I take to mean what I'm trying to feed it isn't a layer, which means the gimp-load-layer operation failed.

My stupidly simple and obstinately not working "script-fu-test.scm" is as follows:

(define (script-fu-test image drawable)
(let*
(
(position -1)
(back-layer nil)
)
(set! back-layer(gimp-file-load-layer RUN-NONINTERACTIVE image "C:/Test/test.png"))
(gimp-image-insert-layer image back-layer -1 position)
[code]......