property read accessor defined via metaClass does not work without parameter list terminator in closure

class Book {
    String title
}

def properties = Collections.synchronizedMap([:])

Book.metaClass.setAuthor = { String value ->
   properties[System.identityHashCode(delegate) + "author"] = value
}
Book.metaClass.getAuthor = {->
   properties[System.identityHashCode(delegate) + "author"]
}

Book.metaClass.getAuthor = {
   properties[System.identityHashCode(delegate) + "author"]
}

book = new Book()
book.author = "foo" // works
println book.author // does not work

Caught: groovy.lang.MissingPropertyException: No such property: author for class: Book
        at Book.getProperty(propertyTest.groovy)
        at foo.run(propertyTest.groovy:15)
        at foo.main(propertyTest.groovy)