Ошибка при использовании gulp.spritesmith

// SASS variables are information about icon's compiled state, stored under its original file name
//
// .icon-home
//   width: $icon-home-width
//
// The large array-like variables contain all information about a single icon
// $icon-home: x y offset_x offset_y width height total_width total_height image_path
//
// At the bottom of this section, we provide information about the spritesheet itself
// $spritesheet: width height image $spritesheet-sprites
$bol-name: 'bol'
$bol-x: 0px
$bol-y: 11px
$bol-offset-x: 0px
$bol-offset-y: -11px
$bol-width: 12px
$bol-height: 12px
$bol-total-width: 28px
$bol-total-height: 23px
$bol-image: 'sprite.png'
$bol: (0px, 11px, 0px, -11px, 12px, 12px, 28px, 23px, 'sprite.png', 'bol', )
$face-name: 'face'
$face-x: 15px
$face-y: 10px
$face-offset-x: -15px
$face-offset-y: -10px
$face-width: 8px
$face-height: 13px
$face-total-width: 28px
$face-total-height: 23px
$face-image: 'sprite.png'
$face: (15px, 10px, -15px, -10px, 8px, 13px, 28px, 23px, 'sprite.png', 'face', )
$mail-name: 'mail'
$mail-x: 0px
$mail-y: 0px
$mail-offset-x: 0px
$mail-offset-y: 0px
$mail-width: 15px
$mail-height: 11px
$mail-total-width: 28px
$mail-total-height: 23px
$mail-image: 'sprite.png'
$mail: (0px, 0px, 0px, 0px, 15px, 11px, 28px, 23px, 'sprite.png', 'mail', )
$twit-name: 'twit'
$twit-x: 15px
$twit-y: 0px
$twit-offset-x: -15px
$twit-offset-y: 0px
$twit-width: 13px
$twit-height: 10px
$twit-total-width: 28px
$twit-total-height: 23px
$twit-image: 'sprite.png'
$twit: (15px, 0px, -15px, 0px, 13px, 10px, 28px, 23px, 'sprite.png', 'twit', )
$spritesheet-width: 28px
$spritesheet-height: 23px
$spritesheet-image: 'sprite.png'
$spritesheet-sprites: ($bol, $face, $mail, $twit, )
$spritesheet: (28px, 23px, 'sprite.png', $spritesheet-sprites, )
 
// The provided mixins are intended to be used with the array-like variables
//
// .icon-home
//   @include sprite-width($icon-home)
//
// .icon-email
//   @include sprite($icon-email)
//
// Example usage in HTML:
//   `display: block` sprite:
//   <div class="icon-home"></div>
//
// To change `display` (e.g. `display: inline-block;`), we suggest using a common CSS class:
//   // CSS
//   .icon {
//     display: inline-block;
//   }
//
//   // HTML
//   <i class="icon icon-home"></i>
@mixin sprite-width($sprite)
  width: nth($sprite, 5)
 
@mixin sprite-height($sprite)
  height: nth($sprite, 6)
 
@mixin sprite-position($sprite)
  $sprite-offset-x: nth($sprite, 3)
  $sprite-offset-y: nth($sprite, 4)
  background-position: $sprite-offset-x  $sprite-offset-y
 
@mixin sprite-image($sprite)
  $sprite-image: nth($sprite, 9)
  background-image: url(#{$sprite-image})
 
@mixin sprite($sprite)
  @include sprite-image($sprite)
  @include sprite-position($sprite)
  @include sprite-width($sprite)
  @include sprite-height($sprite)
 
// The `sprites` mixin generates identical output to the CSS template
//   but can be overridden inside of SASS
//
// @include sprites($spritesheet-sprites)
@mixin sprites($sprites)
  @each $sprite in $sprites
    $sprite-name: nth($sprite, 10)
    .#{$sprite-name}
      @include sprite($sprite)

.class
   +sprites($mail)

sprite($mail)
 
/* или */
 
@return sprite($mail)