diff --git a/docs/lib-sub_str.md b/docs/lib-sub_str.md
index 6ed884f2..ea7304ef 100644
--- a/docs/lib-sub_str.md
+++ b/docs/lib-sub_str.md
@@ -6,7 +6,7 @@ Returns a new string that is a substring of the given string.
 
 - `t` : The original string.
 - `begin` : The beginning index, inclusive.
-- `end` : The ending index, exclusive. If it's omitted, the substring begins with the character at the specified `begin` and extends to the end of this string.
+- `end` : The ending index, exclusive. If it's omitted, the substring begins with the character at the specified `begin` and extends to the end of the original string.
 
 ## Examples