Cascading parameter and report selection

[RCCRookie]

Crysta XI; SQL 2000 DB
I have a cascading Country, State, City parameter and haven't been able to figure out how to account for instances when there is no state or city. My selection formula follows:
{SUPPLIER_MNT_V.COUNTRY} = {?Country/State/City - COUNTRY} and
{SUPPLIER_MNT_V.STATE} = {?Country/State/City - STATE} and
{SUPPLIER_MNT_V.CITY} = {?Country/State/City - CITY}
I tried to include isnull before {SUPPLIER_MNT_V.STATE},{SUPPLIER_MNT_V.CITY} but that didn't work. If I run the parameter without a state or city I get the following error message:
Prompting failed with the following error message:'List of Values failure:fail to get values.[Cause of error: Type mismatch]'. Error source: prompt.dll Error code:0x8004380D

I'd appreciate if someone could point out the obvious (except to me) flaws in my plan.

 

[lbass]

In XI, you must have a value for each parameter, and I don't think there is a way to allow null or blank values in a cascading parameter.

-LB