To reproduce:

public Form1()
{
    InitializeComponent();
    this.radMaskedEditBox1.Mask = "9999999999";
    this.radMaskedEditBox1.MaskType = Telerik.WinControls.UI.MaskType.Standard;
    this.radMaskedEditBox1.PromptChar = Convert.ToChar(" ");

}

private void radButton1_Click(object sender, EventArgs e)
{
    this.radMaskedEditBox1.Clear();
    this.radLabel1.Text = "Value = " + this.radMaskedEditBox1.Value + Environment.NewLine + "Text = " + this.radMaskedEditBox1.Text;
}

Workaround:  set the Value to null as well.