"Cannot be determined because there is no implicit conversion" with ternery if return

I have the following ASP.NET Web Api 2 action with a ternery if return:

[HttpDelete]
public IHttpActionResult Delete()
{
    bool deleted;

    // ...

    return deleted ? this.Ok() : this.NotFound();
}

I receive a

Type of conditional expression cannot be determined because there is no implicit conversion between 'System.Web.Http.Results.OkResult' and 'System.Web.Http.Results.NotFoundResult'

when they both implement IHttpActionResult.

However if I remove the ternery if, the compiler is happy:

if (deleted)
{
    return this.Ok();
}
return this.NotFound();

Why is this?