"The object is not the main asset" error on network.instantiate

I'm encountering an issue with Network instantiation of a prefab that I can't find any information about.

"The object is not the main asset. Only root objecs of a prefab may be used for Instantiating UnityEngine.Network:Instantiate(Object, Vector3, Quaternion, Int32)"

The code looks like this:

var newMissile = Network.Instantiate(missilePrefab, transform.position + transform.TransformDirection(originOffset), launchTube.rotation, 1) as GameObject;

The problem began when I added a child object to an existing prefab. Before this the missile would launch without problems. Now even if I change the prefab in question to another without any children the error still occurs.

Does anyone know what this error actually means and what might be causing it?