determine if value in pivot table

hi,

I have a combobox with a list of value in sheet1. and wen i make a selection
it performs the followig:

sheeta("pivot").range("C6").value = sheets("lookup").range("B4").value

i would like to know if there is any kind of way to add some code to
determine whether or not the value selected via the combobox exists in te
pivot table BEFORE the preceding code is executed. this is to prevent the "x
value does not exist.. rename x to y".

thanks in advance,
geebee