Не работает свойство background в SASS

.mobile 
    min-height: 864px
    padding: 48px 0 79px 0
    background: url('../img/mobile_app.png') center (center / cover) no repeat
    &_item 
      width: 320px
      height: 280px
      margin: 0 auto
      &_1
          background: url('../img/mobile_screen_1.png') center (center / cover) no repeat