How do I search for pages with any value in a given property? I can do a search for pages with a specific value in the property, but I can't figure out how to search for "not empty".

For example, this doesn't work:

var criterias = newPropertyCriteriaCollection
{
  new PropertyCriteria()
  {
    Condition = CompareCondition.NotEqual,
    Name = "MyProperty",
    IsNull = false,
    Type = PropertyDataType.String,
    Value = ""
  }
};

var pages = DataFactory.Instance.FindPagesWithCriteria(PageReference.StartPage, criterias);

An exception is thrown, "The crieria value cannot be null or empty. Set the IsNull property to search for null."

Any ideas?