diff --git a/README.md b/README.md
index a876b5a..29e04f2 100644
--- a/README.md
+++ b/README.md
@@ -235,6 +235,7 @@ Return all keys `>= start` and starts with `prefix`. If `db` is omitted,
 it defaults to `"_default"`.
 
 If `page_size` is specified, up to `page_size` results will be returned.
+`page_size` can not be less than `1`.
 
 The return value of this function is a table `res` where `res[1].key` and `res[1].value`
 corresponds to the first key and value, `res[2].key` and `res[2].value` corresponds to the