C# populate treeview with Linq

im using this code to populate my treeview

var query = data.GroupBy(item => item.Column1)
                .Select(group => group.GroupBy(item => item.Column2))
                .Select(group => group.Select(innerGroup => 
                    innerGroup.GroupBy(item => item.Column3)));

and these foreach code

foreach (var outermostGroup in query)
{
    //add node for outermostGroup's key
    foreach (var middleGroup in outermostGroup)
    {
        //add child node of the above node for middleGroup key
        foreach (var innerGroup in middleGroup)
        {
            //add child node of the above node for innerGroup key
        }
    }
}

but im getting and error trying to get the key from outermostgroup and middleGroup Key, but not from innerGroup, I dont know what's happening. for example.

foreach (var outermostGroup in query)
    {
      var node= new System.Web.Ui.WebControls.TreeNode(outermostGroup.Key)//here doesnt reconize me the key
        foreach (var middleGroup in outermostGroup)
        {
           var node= new System.Web.Ui.WebControls.TreeNode(middleGroup.Key)//here doesnt reconize me the key
            foreach (var innerGroup in middleGroup)
            {
                var node= new System.Web.Ui.WebControls.TreeNode(innerGroup.Key)//here recononize me the Key
            }
        }
    }