Label images from text file with montage command

We can use this command to label each image by its name:

montage -label '%t' -size 512x512 "./img/*.*[120x90]" -geometry +5+5 photo.png

I want to label images by a list of strings, which is written in a text file. Docs say:

If the first character of string is @, the image label is read from a file titled by the remaining characters in the string. Labels in a file are literal, no embedded formatting characters are recognized.

But I could not use a file to provide label. I use a command like this:

montage -label @un.txt -size 512x512 "./img/*.*[120x90]" -geometry +5+5 photo.png 

Above, un.txt is the name of text file in ./img/ path. This is a sample result:

In the result, all images are labeled with the string @un.txt instead of content of un.txt. Also I get this error in terminal:

montage.im6: improper image header `./img/un.txt' @
error/txt.c/ReadTXTImage/429

If I copy un.txt to the current (.) directory and run above command again, I will get this image (there is no label) and below error:

montage.im6: improper image header `./img/un.txt' @
error/txt.c/ReadTXTImage/429. 

montage.im6: not authorized `@un.txt' @
error/property.c/InterpretImageProperties/2959.