Tests failing with Sass 3.4.23

[xzyfer]

Changes to str-slice in Sass 3.4.23 are causing the test suite to fail with

Error: $string: -3 is not a string for `str-length'
        on line 58 of node_modules/sass-true/sass/true/_assert.scss, in `assert-equal'
        from line 42 of tests/api/_fill.scss, in `@content'
        from line 16 of node_modules/sass-true/sass/true/_tests.scss, in `test'
        from line 38 of tests/api/_fill.scss, in `@content'
        from line 15 of node_modules/sass-true/sass/true/_modules.scss, in `test-module'
        from line 1 of tests/api/_fill.scss
        from line 29 of tests/api/_module.scss
        from line 8 of tests/tests.scss
  Use --trace for backtrace.

The failing spec is

    @include test('should work with a negative start <= negative length($list)') {
        @each $value in -3, -4, -9999 {
            $list: 1 2 3;

            @include assert-equal(_fill($list, 'a', $value), 'a' 'a' 'a', $value);
        }
    }

[davidkpiano]

Interesting, I'll take a look.