OR operator within Where clause is not working

Question

I am doing an update using a where clause. Within the where clause I am using an OR condition to get both cases covered. Following is my code:

UPDATE 
    table1
SET
    id = table2.id
FROM table2
where (cast(table2.number as bigint) = table1.number) 
OR (table2.identifier = table1.identifier)

If I execute the update in 2 different update statements, it works. But if I try to do it using OR in the WHERE clause it keeps running forever. One thing that I should mention is I have multiple null values in both the tables for number field as well as for identifier field. But both of these cases cover the total ids that I need to update since the rows where identifier is null there is number available and rows where number is null, an identifier is available. Is there anything I am missing here?