Sass语法错误：＆＃34; ... nth（$ sites，$ i）＆＃34;：expect＆＃34;}＆＃34;之后的无效CSS，＆＃34;：（＆＃34;

Question

我写了一个sass函数，它会抛出一个错误：

Syntax error: Invalid CSS after "...nth($sites, $i)": expected "}", was ": ("

我的代码如下：

@function sites-state-maps($sites, $state){
    $sites-index: 10;
    $state-index: 3;
    @for $i from 1 through $sites-index {
    @return nth($sites, $i): (
        @for $j form 1 through $state-index {
            @return nth($state, $j): state-position($j) site-position($i) @if $j < $state-index {,}
        }
    ) @if $i < $sites-index {,}
    }
    @return false;
}

$background-position: (
    sites-state-maps((aa, bb, cc, dd, ee, ff, gg, hh, ii, jj), (active, hover, current))
);

我不知道这有什么问题？