Example code panics in rustc 1.40.0-nightly

[itsfarseen]

#[derive(OptionalStruct)]
struct Foo {
    meow: u32,
    woof: String,
}

Error message:

error: proc-macro derive panicked
  --> src/domain/items.rs:17:10
   |
17 | #[derive(OptionalStruct)]
   |          ^^^^^^^^^^^^^^
   |
   = help: message: Only optional_name is supported

error: aborting due to previous error