问npm运行生成错误-无法读取未定义的属性'toLowerCase‘

EN

Cannot read property 'toLowerCase' of undefined
CompileError: Begins at CSS selector .btn-white

// buttons
    .btn {
      span {
        color: $white;
      }
      &.btn-white {
        span {
          color: $black;
        }
      }
      &.btn-icon {
        i {
          color: $white;
        }
      }
      &.btn-dark,
      &.btn-outline-dark,
      &.btn-flat-dark {
        color: $gray-600;
      }
      &.btn-dark {
        background-color: $gray-700 !important;
      }
      &.btn-outline-dark {
        border-color: $gray-700;
        color: $gray-600;
      }
      &.btn-flat-dark {
        &:active,
        &:focus {
          background: $gray-700;
        }
      }
      &.btn-white {
        color: $body-color;
      }
    }

// other file
  &.btn-white {
    color: $body-color;
    &:hover,
    &:focus,
    &:active {
      color: $body-color !important;
    }
  }

// for white button (other file)
.btn.btn-white ~ .dropdown-menu {
  .dropdown-item:not(.acitve):hover {
    color: $body-color !important;
  }
  .dropdown-item.active {
    color: $body-color;
  }
}