[julia-users] Why can't I reinterpret a single value to a custom immutable type?

[Jason Merrill]

I have a custom immutable type that wraps an Int16:

julia> immutable DigitSet
           d::Int16
       end

julia> isbits(DigitSet)
true

I was hoping to be able to reinterpret an Int16 as this type, but I get an 
error that confusingly says that the first argument must be a bits type:

julia> reinterpret(DigitSet, Int16(3))
ERROR: reinterpret: expected bits type as first argument
 in reinterpret at essentials.jl:115

I can, however, reinterpret a Vector{Int16} into a Vector{DigitSet}.

julia> reinterpret(DigitSet, Int16[3])
1-element Array{DigitSet,1}:
 DigitSet(3)

Am I seeing a bug here? Or is this expected for some reason?