Underscore & Sp_columns ?!!

Hi friends,

I got troubles when I tried to pass table name to sp_columns via
@local_variable:

declare @tname as char(50)
set @tname = 'TB_MyTable'
exec sp_columns @tname

result of such script is:

TABLE_QUALIFIER ...[skipped]
-----------------------------------
(0 row(s) affected)

But if I will say:

exec sp_columns 'TB_MyTable'

result will be correct (all neccessary data about columns will be provided)

So, looks like if I pass table name which contains underscore via
@local_variable or SP parameter - result will be wrong.
If table name won't contains underscore - everything works fine and result
of script:

declare @tname as char(50)
set @tname = 'MyTable'
exec sp_columns @tname

will be absolutely correct.

Please, anybody can clarify this situation ???

Thanks in advance,

Michael