编译scss出错

Question

用koala编译bootstrap-material-design v4时出错，bootstrap是4.0.0-alpha.3版本。
scss/是编译目录。

Error: Base-level rules cannot contain the parent-selector-referencing character '&'.
on line 106 of scss/mixins/_forms.scss, in `bmd-form-color'
from line 79 of scss/_forms.scss
from line 13 of scss/_core.scss
from line 3 of scss/bootstrap-material-design.scss

源码在此