How do you pass values of a multi-select pick list in a basic S-Control?  Here's my S-Contol:

/a0A/e?CF00N60000001Uy6X={!SessionTEST__c.Client__c}&CF00N60000001UxIh={!SessionTEST__c.Opportunity__c}&00N60000001Uxyd="1"&00N60000001UxyY={!SessionTEST__c.Pose__c}

The code in red is refferences a multi-select picklist, and the values are displayed in the resulting URL:

https://na4.salesforce.com/a0A/e?CF00N60000001Uy6X=Blue+Mountain+Middle+School&CF00N60000001UxIh=Blue+Mountain+Middle+School+Test+Fall+Portaits&00N60000001Uxyd=1&00N60000001UxyY=Puzzle%3BFamily

However, the values "Puzzle" and "Family" are not written to the field.

What am I doing wrong?