diff --git a/_overviews/scala3-book/domain-modeling-tools.md b/_overviews/scala3-book/domain-modeling-tools.md
index f69bad4871..0c947c8080 100644
--- a/_overviews/scala3-book/domain-modeling-tools.md
+++ b/_overviews/scala3-book/domain-modeling-tools.md
@@ -415,7 +415,7 @@ abstract class Pet(name: String):
   def age: Int
   override def toString = s"My name is $name, I say $greeting, and I’m $age"
 
-class Dog(name: String, age: Int) extends Pet(name):
+class Dog(name: String, var age: Int) extends Pet(name):
   val greeting = "Woof"
 
 val d = Dog("Fido", 1)