Less error (unrecognized input) with syntax theme

I’m running into an error (unrecognised input) while building a syntax theme that I can’t find any way to explain.

It tells me that the error is at 278:8 in my base.less, but that is a blank line, it only has a newline.
The code after the newline is:
&.syntax–hr {
color @brown;
}
And deleting that section makes the error go away, but I’m unclear on why less is unhappy with it, or why it fails to give the error location properly.
Any insight would be appreciated.