C# - Can't Initialize A User Control Property Using Eval

I designed a user control. It contains a public property "CurrentValue". When I try to initialize the property using an Eval expression, a null value is assigned.

// In below code, label assignment is OK, RatingNull user control assignment get null

<asp:TemplateField>
<ItemTemplate>
<asp:Label ID="Label1" runat="server" Text='<%# Eval("Difficulty") %>'
<uc1:RatingNull ID="RatingNull1" runat="server" CurrentValue='<%# Eval("Difficulty") %>' />
</ItemTemplate>

If I directly assigned a value (ie, CurrentValue="5"), user control works fine.

public partial class RatingNull : System.Web.UI.UserControl
{
private string _CurrentValue;
public string CurrentValue
{
get { return _CurrentValue; }
set { _CurrentValue = value; }
}
(...)