diff --git a/Source/Constraint.swift b/Source/Constraint.swift
index 859e086..c470065 100644
--- a/Source/Constraint.swift
+++ b/Source/Constraint.swift
@@ -63,7 +63,7 @@ public class Constraint {
 /**
     Used internally to implement a ConcreteConstraint
 */
-internal class ConcreteConstraint: Constraint {
+final internal class ConcreteConstraint: Constraint {
     
     internal override func updateOffset(amount: Float) -> Void {
         self.constant = amount