Sass/Susy mixin issue

Question

I wanted to make a mixin for column spanning containers in SASS using the Susy framework where I could just use an include in a div and use the span-columns such as this:

@mixin container($columns, $ofColumns) {
    @include span-columns($columns,$ofColumns);
}

Then in CSS use it like this:

#foo {
    @include container(4,12);
}

But I get the error in the output css 'Mixin container is missing argument $columns.' What am I doing wrong here?