TypeHandler Override -> SetValue not being called.

I have posted this on StackOverflow but I think it might be a bug, unless I'm missing something super simple.

Am trying to add a custom type handler for an F# Union Type. ParseValue works fine but SetValue does not seem to get called at all.

type PrimaryKey<'x> =
    | Id of int
    | EmptyPrimaryKey

..

type PrimaryKeyHandler<'X>() =
    inherit SqlMapper.TypeHandler<PrimaryKey<'X>>()

    override _.SetValue(param, value) =
        let valueOrNull =
            match value with
            | Id x -> box x
            | EmptyPrimaryKey -> null

        param.DbType <- DbType.Int32 (* Put in as a test , doesn't seem to do anything*)
        param.Value <- valueOrNull

    override _.Parse value =
        if isNull value || value = box DBNull.Value
        then EmptyPrimaryKey
        else Id (value :?> int)

Anything obvious jump out here?

Start with Dapper's SqlMapper.TypeHandler implementation and the registration and parameter-binding path used by the PrimaryKeyHandler example. Reproduce the reported case and trace whether SetValue is reached; done means the handler is invoked and assigns the expected DbType and Value for both union cases.