diff --git a/Perspex/Contract.cs b/Perspex/Contract.cs
index 82edb5af63..1550c098dd 100644
--- a/Perspex/Contract.cs
+++ b/Perspex/Contract.cs
@@ -8,8 +8,14 @@ namespace Perspex
 {
     internal static class Contract
     {
-        public static void Requires<TException>(bool condition) where TException : Exception
+        public static void Requires<TException>(bool condition) where TException : Exception, new()
         {
+#if DEBUG
+            if (!condition)
+            {
+                throw new TException();
+            }
+#endif
         }
     }
 }