diff --git a/oop.tex b/oop.tex
index 0e7da20..d6b020d 100644
--- a/oop.tex
+++ b/oop.tex
@@ -234,8 +234,8 @@ \section{Classes}\label{s:oop-classes}
   new Rectangle(1.5, 0.5)
 ]
 for (let thing of everything) {
-  const a = thing.area(thing)
-  const p = thing.perimeter(thing)
+  const a = thing.area()
+  const p = thing.perimeter()
   console.log(`${thing.name}: area ${a} perimeter ${p}`)
 }
 \end{minted}
diff --git a/src/oop/es6-objects.js b/src/oop/es6-objects.js
index e21d09a..a2db4e8 100644
--- a/src/oop/es6-objects.js
+++ b/src/oop/es6-objects.js
@@ -35,7 +35,7 @@ const everything = [
   new Rectangle(1.5, 0.5)
 ]
 for (let thing of everything) {
-  const a = thing.area(thing)
-  const p = thing.perimeter(thing)
+  const a = thing.area()
+  const p = thing.perimeter()
   console.log(`${thing.name}: area ${a} perimeter ${p}`)
 }