I am trying to use this update statement in order to fill a column Name in a table called MD001 from a table called Patient

MD001(MRN, Name, Address)

Patient(CaseNo, FName, MName, LName,…)

UPDATE QS36F.MD001 m 
SET NAME = 
        (
        SELECT FName || ' ' || Mname || ' ' || LName
        FROM QS36F.PATIENT
        WHERE fname <> '' AND mname <> '' AND lname <> ''
        )
        where  m.MRN  = (SELECT caseno FROM qs36f.patient WHERE caseno = m.MRN) 

I keep getting Result of SELECT more than one row. yet in both tables MRN and CaseNo are primary keys.