Variable defaults in mixins returns errors

[Znegl]

When using a default value for a variable, as following, an error is returned.

.background-image(@img, @repeat: " ", @important: " ") {
    background-image :e(%("url(%s%s%s)", @cdn, @img, @version)) e(@important);
    background-repeat: e(@repeat) e(@important);
}

This returns an error after @repeat: