How to use crypto.createSign with a DiffieHellman private key in node.js?

I'm trying to use a DH private key to sign a message with the node crypto lib. I'm running into an error that I can't seem to fix :{

var crypto = require("crypto");

var bob = crypto.getDiffieHellman("modp17");

bob.generateKeys();

var sign = crypto.createSign("RSA-SHA256");

sign.write("hello world");

var message = sign.sign(bob.getPrivateKey());

Error

140735140705040:error:0906D06C:PEM routines:PEM_read_bio:no start line:../deps/openssl/openssl/crypto/pem/pem_lib.c:703:Expecting: ANY PRIVATE KEY
Error: SignFinal error
    at Sign.sign (crypto.js:398:27)
    at repl:1:18
    at REPLServer.self.eval (repl.js:110:21)
    at repl.js:249:20
    at REPLServer.self.eval (repl.js:122:7)
    at Interface.<anonymous> (repl.js:239:12)
    at Interface.emit (events.js:95:17)
    at Interface._onLine (readline.js:202:10)
    at Interface._line (readline.js:531:8)
    at Interface._ttyWrite (readline.js:760:14)