Hiim using maple 14 and tried to apply the sum of vector's elements, y appears a error (Error, unsupported type of index, n)

a := Vector(5, {(1) = 1, (2) = 2, (3) = 3, (4) = 4, (5) = 5});
> sum(a(n), n = 1 .. 5);

Error, unsupported type of index, n

please need some help

Thanks