Nested Style Rule Parse Error

The CSS Validator is treating a nested style rule as a parse error. Below is the link to the W3C CSS Nesting Module:

      https://www.w3.org/TR/css-nesting-1/

Here is the style rule from the CSS Nesting Module that the CSS Validator rejected:

.foo {
  color: red;

  .bar {
         color: blue;
   }
}