<===> input.scss
@debug(selector-unify("foo", null));

<===> error
Error: $selector2: null is not a valid selector: it must be a string,
       a list of strings, or a list of lists of strings for `selector-unify'
        on line 1 of input.scss
  Use --trace for backtrace.

<===> error-dart-sass
Error: $selector2: null is not a valid selector: it must be a string,
a list of strings, or a list of lists of strings.
  ,
1 | @debug(selector-unify("foo", null));
  |        ^^^^^^^^^^^^^^^^^^^^^^^^^^^
  '
  input.scss 1:8  root stylesheet