Can't access custom field after getting TreeNode via DocumentHelper.GetDocuments()

Hello! I'm getting TreeNode of a custom page type and trying to get it's custom value, which is definetily declared in page type setting, but it returns null.

TreeNode isn't empty and seems got full info in it (including correct page type code name).

Here's my code:

var kenticoTreeNode = DocumentHelper.GetDocuments()
                .Where("NodeAlias", QueryOperator.Equals, "foo")
                .FirstObject;
kenticoTreeNode.GetValue("EktronContentID");

kenticoTreeNode can acquare TreeNodes with different page types, i tried on some and there was error at any page type.

Thanks in advance!