title: sass can be converted to css, but not working with watch command
body: <p>everyone, I am SASS beginner, just started to learn Sass. So, this question may seemed ridiculous to  someones. </p>
  
  <p>Here, I got style.scss</p>
  
  <pre><code>$myColor: #009a82;
  $myString: " some text here ";
  $myFontSize: 13px;
  $myMargin: 0px auto;
  $myWidth: 460px;
  
  h1 {
      color: $myColor;
      margin: 0;
      padding: 0;
  }
  
  #container {
      width: $myWidth;
      margin: $myMargin;
  }
  </code></pre>
  
  <p>when I use command </p>
  
  <blockquote>
    <p>sass style.scss style.css</p>
  </blockquote>
  
  <p>I can get a css file: </p>
  
  <pre><code>h1 {
    color: #009a82;
    margin: 0;
    padding: 0; }
  
  #container {
    width: 460px;
    margin: 0px auto; }
  </code></pre>
  
  <p>However, when I use command</p>
  
  <blockquote>
    <p>sass --watch Sass:style</p>
  </blockquote>
  
  <p>I got :</p>
  
  <pre><code>/*
  Error: Invalid property: "Backtrace:" (no value).
          on line 4 of Sass
  
  1: /*
  2: Errno::ENOENT: No such file or directory @ rb_sysopen - Sass
  3: 
  4: Backtrace:
  5: C:/Ruby21/lib/ruby/gems/2.1.0/gems/sass-3.4.9/lib/sass/plugin/compiler.rb:482:in `read'
  6: C:/Ruby21/lib/ruby/gems/2.1.0/gems/sass-3.4.9/lib/sass/plugin/compiler.rb:482:in `update_stylesheet'
  7: C:/Ruby21/lib/ruby/gems/2.1.0/gems/sass-3.4.9/lib/sass/plugin/compiler.rb:215:in `block in update_stylesheets'
  8: C:/Ruby21/lib/ruby/gems/2.1.0/gems/sass-3.4.9/lib/sass/plugin/compiler.rb:209:in `each'
  9: C:/Ruby21/lib/ruby/gems/2.1.0/gems/sass-3.4.9/lib/sass/plugin/compiler.rb:209:in `update_stylesheets'
  
  Backtrace:
  Sass:4
  C:/Ruby21/lib/ruby/gems/2.1.0/gems/sass-3.4.9/lib/sass/engine.rb:719:in `parse_property'
  C:/Ruby21/lib/ruby/gems/2.1.0/gems/sass-3.4.9/lib/sass/engine.rb:681:in `parse_property_or_rule'
  C:/Ruby21/lib/ruby/gems/2.1.0/gems/sass-3.4.9/lib/sass/engine.rb:647:in `parse_line'
  C:/Ruby21/lib/ruby/gems/2.1.0/gems/sass-3.4.9/lib/sass/engine.rb:525:in `build_tree'
  C:/Ruby21/lib/ruby/gems/2.1.0/gems/sass-3.4.9/lib/sass/engine.rb:544:in `block in append_children'
  C:/Ruby21/lib/ruby/gems/2.1.0/gems/sass-3.4.9/lib/sass/engine.rb:543:in `each'
  C:/Ruby21/lib/ruby/gems/2.1.0/gems/sass-3.4.9/lib/sass/engine.rb:543:in `append_children'
  C:/Ruby21/lib/ruby/gems/2.1.0/gems/sass-3.4.9/lib/sass/engine.rb:396:in `_to_tree'
  C:/Ruby21/lib/ruby/gems/2.1.0/gems/sass-3.4.9/lib/sass/engine.rb:368:in `_render_with_sourcemap'
  C:/Ruby21/lib/ruby/gems/2.1.0/gems/sass-3.4.9/lib/sass/engine.rb:285:in `render_with_sourcemap'
  C:/Ruby21/lib/ruby/gems/2.1.0/gems/sass-3.4.9/lib/sass/plugin/compiler.rb:490:in `update_stylesheet'
  C:/Ruby21/lib/ruby/gems/2.1.0/gems/sass-3.4.9/lib/sass/plugin/compiler.rb:215:in `block in update_stylesheets'
  C:/Ruby21/lib/ruby/gems/2.1.0/gems/sass-3.4.9/lib/sass/plugin/compiler.rb:209:in `each'
  C:/Ruby21/lib/ruby/gems/2.1.0/gems/sass-3.4.9/lib/sass/plugin/compiler.rb:209:in `update_stylesheets'
  C:/Ruby21/lib/ruby/gems/2.1.0/gems/sass-3.4.9/lib/sass/plugin/compiler.rb:293:in `watch'
  C:/Ruby21/lib/ruby/gems/2.1.0/gems/sass-3.4.9/lib/sass/plugin.rb:108:in `method_missing'
  C:/Ruby21/lib/ruby/gems/2.1.0/gems/sass-3.4.9/lib/sass/exec/sass_scss.rb:370:in `watch_or_update'
  C:/Ruby21/lib/ruby/gems/2.1.0/gems/sass-3.4.9/lib/sass/exec/sass_scss.rb:50:in `process_result'
  C:/Ruby21/lib/ruby/gems/2.1.0/gems/sass-3.4.9/lib/sass/exec/base.rb:52:in `parse'
  C:/Ruby21/lib/ruby/gems/2.1.0/gems/sass-3.4.9/lib/sass/exec/base.rb:19:in `parse!'
  C:/Ruby21/lib/ruby/gems/2.1.0/gems/sass-3.4.9/bin/sass:13:in `&lt;top (required)&gt;'
  C:/Ruby21/bin/sass:23:in `load'
  C:/Ruby21/bin/sass:23:in `&lt;main&gt;'
  */
  body:before {
    white-space: pre;
    font-family: monospace;
    content: "Error: Invalid property: \"Backtrace:\" (no value).\A         on line 4 of Sass\A \A 1: /*\A 2: Errno::ENOENT: No such file or directory @ rb_sysopen - Sass\A 3: \A 4: Backtrace:\A 5: C:/Ruby21/lib/ruby/gems/2.1.0/gems/sass-3.4.9/lib/sass/plugin/compiler.rb:482:in `read'\A 6: C:/Ruby21/lib/ruby/gems/2.1.0/gems/sass-3.4.9/lib/sass/plugin/compiler.rb:482:in `update_stylesheet'\A 7: C:/Ruby21/lib/ruby/gems/2.1.0/gems/sass-3.4.9/lib/sass/plugin/compiler.rb:215:in `block in update_stylesheets'\A 8: C:/Ruby21/lib/ruby/gems/2.1.0/gems/sass-3.4.9/lib/sass/plugin/compiler.rb:209:in `each'\A 9: C:/Ruby21/lib/ruby/gems/2.1.0/gems/sass-3.4.9/lib/sass/plugin/compiler.rb:209:in `update_stylesheets'"; }
  </code></pre>
  
  <p>Anyone can tell me why? 
  Do I use the "watch" command in a wrong way? </p>
  
  <p>Many thanks. </p>