Possible problem with isnumeric function

If an integer is a number then why the following does not work:

declare @a char(1)

select @a="."

select convert(int, case when (isnumeric(@a)=1) then @a else "0" end)

While the follwoing works: 

declare @a char(1)

select @a="."

select convert(int, convert (numeric,case when (isnumeric(@a)=1) then @a else "0" end))

0

My question is: should isnumeric(".") return 1; as it does?