Dependent Drop Down Lists

 Hi,  I have 4 Dependent Drop down lists  on my form.  They work fine when selected in order but when the user changes their mind and changes any of the parent drop downs , the following error occurs:
 
'ddl....' has a SelectedIndex which is invalid because it does not exist in the list of items.
Parameter name: value Is there anyway around this??
 
Thanks
Fiorano
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