Error when allocating within a select type statement

Can anybody explain why the following code is not permitted?

subroutine Allocation1(Vec)
    class(*), allocatable, intent(out)      :: Vec(:)

    select type(Vec)
    type is(real(8))
        allocate(Vec(10)); Vec = 0.D0
    type is(complex(8))
        allocate(Vec(10)); Vec = (0.D0,0.D0)
    type is(integer)
        allocate(Vec(10)); Vec = 0
    endselect
endsubroutine Allocation1