title: Simple extract with Lambdaj causes "Unable to convert placeholder null"
body: <p>I have a simple pojo:</p>
  
  <pre><code>class UserId {
    String ssn;
    String otsId;
    Integer actorId;
  
    public UserId(String ssn, String otsId, Integer actorId) {
      this.ssn = ssn;
      this.otsId = otsId;
      this.actorId = actorId;
    }
  
    @Override
    public String toString() {
      return "[otsId=" + otsId + ", ssn=" + ssn + ", actorId=" + actorId + "]";
    }
  }
  </code></pre>
  
  <p>And I want to extract for example all the <code>ssn</code> values to a <code>List&lt;String&gt;</code> from there. So just as an example I write:</p>
  
  <pre><code>public class UserIdTest {
  
    public static void main(String[] args) {
      List&lt;UserId&gt; list = new ArrayList&lt;UserId&gt;();
      list.add(new UserId("111111-1111", "12345678", new Integer(234589235)));
      list.add(new UserId("111111-1111", "12345678", new Integer(234589235)));
      list.add(new UserId("111111-1111", "12345678", new Integer(234589235)));
  
      getSsnList(list);
    }
  
    private static List&lt;String&gt; getSsnList(List&lt;UserId&gt; users) { 
      return extract(users, on(UserId.class).ssn); 
    }
  }
  </code></pre>
  
  <p>and LambdaJ throws:</p>
  
  <pre><code>Exception in thread "main" ch.lambdaj.function.argument.ArgumentConversionException: Unable to convert the placeholder null in a valid argument
  at ch.lambdaj.function.argument.ArgumentsFactory.actualArgument(ArgumentsFactory.java:76)
  at ch.lambdaj.function.convert.ArgumentConverter.&lt;init&gt;(ArgumentConverter.java:29)
  at ch.lambdaj.Lambda.extract(Lambda.java:1035)
  at UserIdTest.getSsnList(UserIdTest.java:23)
  at UserIdTest.main(UserIdTest.java:20)
  </code></pre>
  
  <p>This seems like a really basic operation so what am I missing here?</p>