Is the Koala compiler throwing an error?

Question

Sass

I just started using Koala, maybe I don’t understand something, but when compiling sass, I get the error " Error: Invalid CSS after "97%": expected expression (eg 1px, bold), was ";" "
All code:

#main {
  width: 97%;

  p, div {
    font-size: 2em;
    a { font-weight: bold; }
  }

  pre { font-size: 3em; }
}