Question : how to add two textboxes in access form to display in another textbox

I have a form in access 2000. I have a 3 unbound textboxes and one command button.
the datatype of textboxes is number.
If I code command button : me.textbox4 = me.textbox0 + me.textbox2
the equation works except when a number is not inputted into textbox0 or textbox2.
Can you tell me why this is?
and how can I have the number add anyways - even if one of two textboxes is empty?
thank you

Answer : how to add two textboxes in access form to display in another textbox

In the above code, the NZ function changes "nulls" to zeros, which can be added for a sum to be shown in the texbox4.