SASS: Only properties may be nested in properties. Why?

I have the following SASS script, but PhpStorm says it's "Ilegal nesting: Only properties may be nested beneath properties." What's wrong with nesting element stylings? I come from LESS scripting, where this works just fine:

header

  nav
    float: left

      ul
        list-style: none
        margin: 0

        li
          float: left
          margin: 2px 5px

          a
            color: $blueish
            font-size: 0.8rem

How can I solve this?