grunt-contrib-less cannot find source files even though path is correct

I'm trying to get grunt-contrib-less to compile a less file for me. I have the following gruntfile:

less: {
        files: {
            "httpdocs/static/assets/css/result.css": "httpdocs/static/assets/css/dashboard.less"
        }
    },

The paths are definitely correct, I've triple checked that those files exist, yet, when I run grunt I get the following message:

Running "less:files" (less) task
>> Destination not written because no source files were provided.

What am I missing from my gruntfile to make it properly compile the less file? I'm tearing my hair out because I know it's probably something really simple but I can't figure it out.