nvl function fail with NullPointerException if the two paramtype are different

Hi all

when i user nvl and case when fucntion, it behaviors like the following.which is weird to me, and makes me headache everytime. i need to check where the nullpointexception throws out when it hadppens,So can the reasons more friendly to users,

select nvl(cast('2019-05-10 11:11:11,111' as timestamp),'2019-05-10 11:11:11,111');
Error: Error while compiling statement: FAILED: NullPointerException null (state=42000,code=40000)