在同级导入文件中扩展类无效

我想用Sass的 @extend

在里面 _title.scss

.title { font-family: "Open Sans"; }

_user-content.scss

.user-content { h1, h2, h3, h4, h5, h6 { @extend .title; } }

在里面 screen.scss :

@import "_title";
@import "_user-content";

运行时 gulp-sass

[13:15:02] gulp-notify: [Gulp] src/assets/styles/base/user-content/_user-content.scss
Error: ".user-content h1" failed to @extend ".title".
       The selector ".title" was not found.
       Use "@extend .title !optional" if the extend should be able to fail.
        on line 1 of src/assets/styles/base/user-content/_user-content.scss
>> .user-content { h1, h2, h3, h4, h5, h6 { @extend .title; } }
   -----------------------------------------^

在我看来这似乎是正确的,但这是一个错误。我做错了什么?