asp.net text box not taking null value

i am new to asp.net ,i have two textboxes and in one user will enter value which will get added to the other automatically generated textbox value.But is user is not entering any value it is showing error.Any solution to solve this

 

It could be some other problem, as when you don't enter a value into the text box, the Text property returns an empty string and not a null. Can you please post the code snippet so that we get more idea about what you are trying to do.