Cast NULL into Timestamp?

I have a table

create table temp
(tempdate    timestamp,tempname    varchar(10)
);

And I tried to insert the following:

insert into table temp (tempname, tempdate)
select distinct 'tempname', null from some_other_relevant_table;

And I got an error that says "column "tempdate" is of type timestamp ... but 
expression is of type text... will need to rewrite or cast the expression".

I really need the distinct. Is there anyway to cast this NULL into a 
timestamp or any other workarounds?


Thanks

Wei