diff --git a/docs/utils/immutable.md b/docs/utils/immutable.md
index 3b4ce2dc..a692dc40 100644
--- a/docs/utils/immutable.md
+++ b/docs/utils/immutable.md
@@ -181,7 +181,7 @@ class MyStore {
   }
 
   onUpdateProp1(newProp1) {
-    this.data = this.data.set(prop1: newProp1);
+    this.data = this.data.set('prop1', newProp1);
   }
 }