Undo last commad button

sorry ok this is the error Method"undo"of obect_'application'failed. error 1004

My code for the command buttons are for example

TextBox1.Value = TextBox1.Value & " check"

TextBox1.Value = TextBox1.Value & " testing"

so what happens is whatever is written in the text box stays and "check" is then written in the text box.

so what I need the undo command button to do is delete the last entry in the textbox. Also the text box was created using the control toolbox. hope this helps