Oracle 10g - Modify select CASE

Experts,

I want to modify the query below.

Current behaviour:
SELECT PROBILL from TABLE A
IF PROBILL is NULL  SELECT MIN value from TABLEB

I have found that sometimes TABLEB is also NULL. If so, I want to return :Ordnum from the input.

SELECT CASE 
  WHEN PROBILL IS NULL
  THEN
    (Select MIN(myvalue) 
    From TABLEB 
    WHERE ORD_NUM = :Ordnum
    and COMP_CODE = :Comp)
  ELSE PROBILL
  END
FROM (SELECT MIN(WAY_BILL_NUM) PROBILL
  FROM TABLEA
  WHERE ORD_NUM = :Ordnum
  and COMP_CODE = :Comp)

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