sassc-rails is having hard time parsing css rgb function

Question

I have the following css class in application.css

.shadow {
  box-shadow: 0 10px 30px 0 rgb(3 32 20/.50)!important;
}

When try to precompile assets the sass parser throws this error:

rake aborted!
remote:        SassC::SyntaxError: Error: Function rgb is missing argument $green.
remote:                on line 76 of stdin
remote:        >>   box-shadow: 0 10px 30px 0 rgb(3 32 20/.50)!important;

If I put commas rgb(3, 32, 20/.50) the parser sees no error but the problem is that the version with commas is not a valid css.

Does anyone have any idea where the error might be?