Julia: no method `copy(::Missing)`

[denius]

ERROR: MethodError: no method matching copy(::Missing)

I suppose this can be fixed by adding copy(::Missing) = missing somewhere.

I discovered this when trying to use "empty" sparse matrices -- non-zero patterns with no values, like sparse(transpose(similar(sprand(Bool,3,3,0.2), Missing)))

[JeffBezanson]

Rather, I think the sparse code should not be calling copy here; it should not require the elements to have copy methods.