diff --git a/readme.md b/readme.md
index 153993a..1568f2b 100644
--- a/readme.md
+++ b/readme.md
@@ -245,6 +245,8 @@ The `default` value set in `boolean` flags take precedence over `booleanDefault`
 
 _Note: If used in conjunction with `isMultiple`, the default flag value is set to `[]`._
 
+__Caution: Explicitly specifying undefined for `booleanDefault` has different meaning from omitting key itself.__
+
 Example:
 
 ```js