I just installed Breakpoint, added 

```
require "breakpoint"

```

to my config.rb, and defined these breakpoints

```
$mobileMedium : 480px;
$tabletVertical : 768px;
$desktopNormal : 1024px;
$desktopWide : 1200px;

```

to my \_breakpoint.scss.

When I tried to use one of the breakpoints, like this;

```
@include breakpoint($mobileMedium) {
…
}

```

terminal told me

```
error sass/screen.scss (Line 27: Undefined mixin 'breakpoint'.)

```

why is that?