Data in drop down list box cannot be read when visibility is set to false.

Hi everyone. I have this really small problem but I can't seem to find a solution.
I am making an inventory system in which I have to get the unit price of an item by using sql data sources. I used a drop down list box to get it. I triggered the computation of the items during the 'SelectedINdexChanged' event of another dropdown listbox which is the Quantity.
Everything goes well if the drop down list boxes visibility are set to true. But when it is et to false, I get an error message like this:
Input string was not in a correct format. Description: An unhandled exception occurred during the execution of ...