Macro is not working

I am new to VBA, so please bear with me.As most of you probably know,the columns size in excel do not automatically adjust themselves, i.e. you have to manually re-adjust them. Having learned VBA myself, I have created a change event Marco so that when I type anything in a cell, the columns widths are adjusted automatically. It is a single line code. It works fine, but I have encountered another problem. The UNDO button or function has now been disabled. I am unable to undo anything in a cell, but I do not know the reason and how do I overcome this problem? Yes, if I remove the code, the undo works.Thank you for any comment!