diff --git a/Mac/Preferences/PreferencesWindowController.swift b/Mac/Preferences/PreferencesWindowController.swift
index 825078e23..ec35176ee 100644
--- a/Mac/Preferences/PreferencesWindowController.swift
+++ b/Mac/Preferences/PreferencesWindowController.swift
@@ -27,7 +27,7 @@ private struct ToolbarItemIdentifier {
 	static let Advanced = "Advanced"
 }
 
-class PreferencesWindowController : NSWindowController, NSToolbarDelegate {
+final class PreferencesWindowController : NSWindowController, NSToolbarDelegate {
 	
 	private let windowWidth = CGFloat(512.0) // Width is constant for all views; only the height changes
 	private var viewControllers = [String: NSViewController]()