Winform Masked Textbox causes ModifiedChanged event to not fire

Question

I have been struggling with this one issue and just can't seen to find an answer. If I set a mask value the ZipcodeTextbox MaskedTextbox control it will result in the event not firing. In the code below I can comment out the mask setting and the event will fire. I have even tried to set the mask on the ZipcodeTextbox the same as the SupportPhoneTextbox and still the event will not fire.

Any suggestions or pointing me towards an answer is greatly appreciated.

// 
// SupportPhoneTextbox
// 
this.SupportPhoneTextbox.Font = new System.Drawing.Font("Microsoft Sans Serif", 10F, System.Drawing.FontStyle.Regular, System.Drawing.GraphicsUnit.Point, ((byte)(0)));
this.SupportPhoneTextbox.Location = new System.Drawing.Point(410, 46);
this.SupportPhoneTextbox.Mask = "(999) 000-0000";
this.SupportPhoneTextbox.Name = "SupportPhoneTextbox";
this.SupportPhoneTextbox.Size = new System.Drawing.Size(114, 23);
this.SupportPhoneTextbox.TabIndex = 31;
this.SupportPhoneTextbox.TextMaskFormat = System.Windows.Forms.MaskFormat.ExcludePromptAndLiterals;
this.SupportPhoneTextbox.TextChanged += new EventHandler(SupportPhoneTextbox_ModifiedChanged);
// 
// ZipcodeTextbox
// 
this.ZipcodeTextbox.Font = new System.Drawing.Font("Microsoft Sans Serif", 10F, System.Drawing.FontStyle.Regular, System.Drawing.GraphicsUnit.Point, ((byte)(0)));
this.ZipcodeTextbox.Location = new System.Drawing.Point(74, 126);
this.ZipcodeTextbox.Name = "ZipcodeTextbox";
this.ZipcodeTextbox.Mask = "00000-9999";
this.ZipcodeTextbox.Size = new System.Drawing.Size(83, 23);
this.ZipcodeTextbox.TabIndex = 34;
this.ZipcodeTextbox.TextMaskFormat = System.Windows.Forms.MaskFormat.ExcludePromptAndLiterals;
this.ZipcodeTextbox.ModifiedChanged += new EventHandler(ZipcodeTextbox_ModifiedChanged);