diff --git a/Frameworks/RSTree/RSTree/Node.swift b/Frameworks/RSTree/RSTree/Node.swift
index 618bad9d4..bf2270253 100644
--- a/Frameworks/RSTree/RSTree/Node.swift
+++ b/Frameworks/RSTree/RSTree/Node.swift
@@ -8,6 +8,8 @@
 
 import Foundation
 
+// Main thread only.
+
 public final class Node: Hashable {
 	
 	public weak var parent: Node?
@@ -17,7 +19,6 @@ public final class Node: Hashable {
 	public var childNodes: [Node]?
 	public let hashValue: Int
 	private static var incrementingID = 0
-	private static var incrementingIDLock = NSLock()
 
 	public var isRoot: Bool {
 		get {
@@ -63,14 +64,14 @@ public final class Node: Hashable {
 	}
 
 	public init(representedObject: AnyObject, parent: Node?) {
-		
+
+		precondition(Thread.isMainThread)
+
 		self.representedObject = representedObject
 		self.parent = parent
 
-		Node.incrementingIDLock.lock()
 		self.hashValue = Node.incrementingID
 		Node.incrementingID += 1
-		Node.incrementingIDLock.unlock()
 	}
 	
 	public class func genericRootNode() -> Node {