Nested media queries break postCSS source mapping

[Jexordexan]

Consider the following Less:

.nav-placeholder {
  height: 70px;
  @media (min-width: 700px) {
    height: 50px;
  }
}

This is valid Less, but throws an error in the console when map: true option is set in the postCSS config:

Fatal error: Invalid mapping: {"generated":{"line":1,"column":4349},"source":"../partials/nav.less","original":{"line":1,"column":-1},"name":null}

Curiously, the following works fine:

.nav {
  .nav-placeholder {
    height: 70px;
  }
  @media (min-width: 700px) {
    .nav-placeholder
      height: 50px;
    }
  }
}

Notice that the media query is still nested but the rules are contained in a selector block.

Not sure if this is a bug for the normal postcss repo but I'll see where this goes!