Getting warning: Null value is eliminated by an aggregate or other SET operation

Getting warning: Null value is eliminated by an aggregate or other SET
operation

I have this schema
create table t(id int, d date)
insert into t (id, d) values (1, getdate()),
(2, NULL)
When doing
declare @mindate date
select @mindate = min(d) from t2
I get the warning
Null value is eliminated by an aggregate or other SET operation
Why and what can I do about it?