How to reproduce: this.radMaskedEditBox1.Font = new System.Drawing.Font("Segoe UI", 20F); this.radMaskedEditBox1.MaskType = Telerik.WinControls.UI.MaskType.Standard; this.radMaskedEditBox1.Mask = "#####-######"; this.radMaskedEditBox1.Value = "47997006886"; Workaround: this.radMaskedEditBox1.Font = new System.Drawing.Font("Segoe UI", 20F); this.radMaskedEditBox1.MaskType = Telerik.WinControls.UI.MaskType.Standard; this.radMaskedEditBox1.Mask = "#####-######"; this.radMaskedEditBox1.Text = "47997006886";