"filter: alpha(@ieopacity);" yields "filter: alpha([object Object])"

[Searle]

i tried:

.opacity( @opacity: 0.5 ) {
    @ie-opacity: 100 * @opacity;
    filter: alpha(opacity=@ie-opacity);
}

and got
filter: alpha(opacity=[object Object]);:

However, there is a work-around:

@ie-opacity: 100 * @opacity;
filter: e(%("alpha(opacity=%s)", @ie-opacity));