IsNull() on bigint's min value?

Question

Why does the following expression in SQL Server return -9223372036854775808 and not 123?

I am calling this from a stored proc where I can't pass null parameters

declare @t bigint;
set @t = -9223372036854775808;  --min value for bigint / long
select ISNULL(@t, 123)

Answer 1

Because:

IF @t IS NOT NULL
  PRINT @t
ELSE
  PRINT 123

Being negative doesn't mean the value is NULL. NULL is the lack of any value at all.

Answer 2

Because @t is not null.

What made you think that the most negative value for a bigint would be interpreted as null?