mixin中的插值问题

Question

我在$ flex元素上进行插值时遇到了一些麻烦，我得到的答案是： [错误：“@include”后的CSS无效：预期的标识符是“＃{$ flex}（）;”]

@mixin medQ-flex(
  $top,
  $right,
  $bottom,
  $left,
  $flex,
  $media: $brkpoint-sm){
  @if $media == $brkpoint-sm {
    @media (min-width: $brkpoint-sm) and (max-width: $brkpoint-md - 1){
      margin: $top $right $bottom $left;
      @include #{$flex}();
      @content;
     }
    }
   }