Any ideas why the following script is producing the error

'umbraco.MacroEngines.DynamicNodeList' does not contain a definition for 'Random'

Source is a contentPicker property, and Limit is a numeric property

Thanks in advance for any assistance

@inherits umbraco.MacroEngines.DynamicNodeContext

@{
  if (Model.HasProperty("featuredWidgets") && !String.IsNullOrEmpty(Model.FeaturedWidgets.ToString())) {
    foreach (var item in Model.FeaturedWidgets) {
      var node = Model.NodeById(item.InnerText);
      var source = Model.NodeById(node.Source);
      if(source.NodeTypeAlias == "TestimonialArea")
      {
        <div class="widget testimonial">
          <h3>@node.Name</h3>
          @foreach(var testimonial in source.Children.Where("Visible").Random(source.Limit)) {
            @testimonial.BodyText
          }
        </div>
      }
    }
  }
}