I'm having a brain freeze for the error, generating in the following code;

It seems that:

var firstImageUrl = images.First();

Goes wrong because the Extension Method First (LINQ) cant be found.
Anyone?

@inherits umbraco.MacroEngines.DynamicNodeContext 
@using umbraco.MacroEngines; 
@using System.Linq; 
@using System.Xml.Linq
@using umbraco.cms.businesslogic.media;
@{
    var csv = uQuery.GetCurrentNode().GetProperty<string>("articleImages");
    var images = uQuery.GetMediaByCsv(Model.articleImages);
    
    var firstImageUrl = images.First();
    var urlMediumFirst = String.Format("/ImageGen.ashx?image={0}&width=300&height=300&constrain=true",firstImageUrl);
    
    <div id="imageGallery">
        <div id="imageGalleryPlaceHolder"><div id="imageGalleryContent"><img id="mainProductImage"  src="@urlMediumFirst" border="0"/></div></div>
        <div id="imageGalleryButtons">
            @foreach (Media img in images)
            {
                var imageUrl = img.GetImageUrl();
                var urlMedium = String.Format("/ImageGen.ashx?image={0}&width=300&height=300&constrain=true",imageUrl);
                var urlSmall = String.Format("/ImageGen.ashx?image={0}&width=50&height=50&constrain=true",imageUrl);
                    
                <a href="#" rel="@urlMedium" class="galleryImage"><img  src="@urlSmall" border="0"/></a> 
            }
        </div>
    </div>
}