diff --git a/manuscript/12-Proxies-and-Reflection.md b/manuscript/12-Proxies-and-Reflection.md
index 9ed01899..e6f58852 100644
--- a/manuscript/12-Proxies-and-Reflection.md
+++ b/manuscript/12-Proxies-and-Reflection.md
@@ -103,7 +103,7 @@ let proxy = new Proxy(target, {
         // ignore existing properties so as not to affect them
         if (!trapTarget.hasOwnProperty(key)) {
             if (isNaN(value)) {
-                throw new TypeError("Property must be a number.");
+                throw new TypeError("Property value must be a number.");
             }
         }