I have a blob of code, that works fine and looks to be standar razor code... But when using this within a typedContext context it fails

The working code outside of the TypedContent is this :

if (@fieldset.HasValue("youtube")) {
    string myString = @fieldset.GetValue("youtube");
    int charPositionStart = myString.IndexOf("=");
    int lengthOfText = 11;
    myString = myString.Substring(charPositionStart+1, lengthOfText);
    var myBg = "http://img.youtube.com/vi/" + @myString + "/maxresdefault.jpg";

    ---- HTML stuff ----
}

if (@boxItem.HasValue("youtube")) {
    string myString = @boxItem.GetPropertyValue("youtube");
    int charPositionStart = myString.IndexOf("=");
    int lengthOfText = 11;
    myString = myString.Substring(charPositionStart+1, lengthOfText);
    var myBg = "http://img.youtube.com/vi/" + @myString + "/maxresdefault.jpg";

    ---- HTML stuff -----

}

Compiler Error Message: CS0266: Cannot implicitly convert type 'object' to 'string'. An explicit conversion exists (are you missing a cast?)

on this line:

string myString = @boxItem.GetPropertyValue("youtube");