This's my susy config
$susy: (
    flow: ltr,
    math: fluid,
    output: float,
    gutter-position: right,
    container: 1000px,
    container-position: center,
    columns: 12,
    gutters: .15,
    column-width: false,
    global-box-sizing: content-box,
    last-flow: to,
    debug: (
            image: show,
            color: rgba(#66f, .25),
            output: overlay,
            toggle: top right,
    ),
    use-custom: (
            background-image: true,
            background-options: false,
            box-sizing: true,
            clearfix: false,
            rem: true,
    )
);
Test code:
.test {
    @include gutter();
    margin-bottom: gutters();
}
After compiling they'r return nothing...
Or
.test {
      margin-left: span(2) + gutters(2);
}
Displays an error: Invalid null operation: "15.75092% plus null"
What's the problem?