When I type, kubeadm init into the terminal, I get and Invalid Value error:

root@kevin-Lenovo-Y430P:/home/kevin# kubeadm init 
[kubeadm] WARNING: kubeadm is in alpha, please do not use it for production clusters.
[preflight] Running pre-flight checks
[init] Using Kubernetes version: v1.5.4
discovery: Invalid value: null: exactly one discovery strategy can be provided
root@kevin-Lenovo-Y430P:/home/kevin# 

When I use kubeadm init -h find the output is:

--discovery discovery  The discovery method kubeadm will use for connecting nodes to the master (default unknown)

So what are the discovery methods that I can choose?