diff --git a/doc/Type/Str.pod6 b/doc/Type/Str.pod6
index ccdb4d6dc..23e60ee5d 100644
--- a/doc/Type/Str.pod6
+++ b/doc/Type/Str.pod6
@@ -1048,6 +1048,7 @@ A version of C<substr> that returns a L<Proxy|/type/Proxy> functioning as a
 writable reference to a part of a string variable. Its first argument, C<$from>
 specifies the index in the string from which a substitution should occur, and
 its last argument, C<$length> specifies how many characters are to be replaced.
+If not specified, C<$length> defaults to the length of the string.
 
 For example, in its method form, if one wants to take the string C<"abc">
 and replace the second character (at index 1) with the letter C<"z">, then