Sass can't compile CSS due to rgb syntax?

Question

Sass

I'm trying to compile SCSS source to CSS, but I've encountered an error

Error: wrong number of arguments (given 1, expected 3) for `rgb'

According to her, you need to pass the missing values ​​​​to the rgb function BUT they are!
This is what a variable looks like, in which there are supposedly not enough values: I call it like this:
--bg-color: rgba(3 3 3 / 40%);

.bg{
  background-color: var(--bg-color);
 }

SASS version - Sass 4.0.0.alpha.1 (Bleeding Edge)

Copy to browser - everything works. What am I doing wrong?