Update with sub select - How to handle NULL values?

I'm trying an update with a conditional sub-select which could return null...

UPDATE 
aTable SET 
aColumn = 
(   
    SELECT TOP 1    
        CASE 
            WHEN bTable.someColumn = 1 THEN someValue1 
            WHEN bTable.someColumn = 2 THEN someValue2 
            ELSE someValue3
        END  
    FROM         
        bTable
    WHERE
        bTable = @someCriteria
    ORDER BY
        someSortColumn
) WHERE 
aTable.id = @someId;

If the "bTable = @someCriteria" clause causes no results to be returned from the SELECT, it attempts to insert a NULL into "aColumn", which in this case is a NOT NULL column.

Question

How do I get it to simply leave "aColumn" alone in this circumstance?

Many thanks.