diff --git a/docs/withAttr.md b/docs/withAttr.md
index 9d8ccd1f..53dfd9c9 100644
--- a/docs/withAttr.md
+++ b/docs/withAttr.md
@@ -39,7 +39,7 @@ m.mount(document.body, Component)
 Argument    | Type                 | Required | Description
 ----------- | -------------------- | -------- | ---
 `attrName`  | `String`             | Yes      | The name of the attribute or property whose value will be used
-`callback`  | `any -> Boolean?`    | Yes      | The callback
+`callback`  | `any -> undefined`   | Yes      | The callback
 `thisArg`   | `any`                | No       | An object to bind to the `this` keyword in the callback function
 **returns** | `Event -> undefined` |          | An event handler function