Unable to document Base methods from another module?

I've found that attempts to document methods from Base while in another module fail:

``` julia
julia> @doc """
       Foo
       """ ->
       function Base.sum(a::Vector{Int})
           return NaN
       end
ERROR: `doc` has no method matching doc(::Module, ::Void, ::MD, ::Expr)
 in anonymous at docs.jl:40
```

Is this a bug? Or am I using `@doc` improperly?
