Terrain addTreeInstance crash

I seem to have a problem with creating trees on a terrain object using terrain.AddTreeInstance(instance); function because it crashes unity on play.

I got multiple terrain objects generating at runtime and I'm trying to add one tree to the corner of each terrain with this code:

NOTE: I'm creating the trees before actually setting the terrainData to the terrain so it does update the terrain after my trees get added.

 var tmp : TreeInstance = new TreeInstance();
 tmp.heightScale = 1.0f;
 tmp.widthScale = 1.0f;
 tmp.prototypeIndex = 0;
 tmp.color = Color.white;
 tmp.position = new Vector3(0.0f,heightMap[0,0],0.0f);
 terrain.AddTreeInstance(tmp);

Also I have added one tree prefab to the treeprototypes array earlier in the code and it does show up in the inspector if I do not create trees this way.

Does anyone else experience this or am I using the AddTreeInstance function wrong?