The documentation for genre says that the values expected is of type text or URL. However, the JSON-LD Context file says:

genre: {
    @id: "schema:genre",
    @type: "@id"
},

which forces the value to be always a URL. This may also lead to syntactically incorrect URL-s, and is certainly not in line with the documentation...

@BigBlueHat @RichardWallis