diff --git a/readme.md b/readme.md
index 96023c5..dd7914b 100644
--- a/readme.md
+++ b/readme.md
@@ -93,9 +93,9 @@ Note: `is.number(NaN)` returns `false`. This intentionally deviates from `typeof
 
 #### Built-in types
 
-##### .array(value, assertion)
+##### .array(value, assertion?)
 
-Returns true if `value` is an array and all of it's items match the assertion.
+Returns true if `value` is an array and all of its items match the assertion (if provided).
 
 ```js
 is.array(value) // validate `value` is an array