.less mixin guard won't accept @media in expression

Question

I'm trying to create a mixin guard, as seen on http://lesscss.org/features/ , where they do mixin(@a) when (@media = mobile) { ... }

However, when I do mixin() when (@media = mobile) { ... } or mixin(@a) when (@media = mobile) { ... }, less is saying it's expecting a ) at @media's @ sign and that it's expecting an "expression" at the closing ). Conversely, if I do mixin(@a) when (@a = 5) { ... }, less is happy.

Why is this, and how can I get @media to work in my mixin's guard?