v4 Sass compile error on flex

Created by: JanStevens

Hello,

When compiling v4 bootstrap flex I get the following syntax error:

Sass::SyntaxError:         Sass::SyntaxError: ".col-xs-1" failed to @extend "%grid-column-float-xs".
        The selector "%grid-column-float-xs" was not found.
        Use "@extend %grid-column-float-xs !optional" if the extend should be able to fail.

Regards,