diff --git a/runtime/doc/builtin.txt b/runtime/doc/builtin.txt
index b14851c..1b7c6dc 100644
--- a/runtime/doc/builtin.txt
+++ b/runtime/doc/builtin.txt
@@ -10127,8 +10127,8 @@
 		When {charidx} is present and TRUE, {idx} is used as the
 		character index in the String {string} instead of as the byte
 		index.
-		An {idx} in the middle of a UTF-8 sequence is rounded upwards
-		to the end of that sequence.
+		An {idx} in the middle of a UTF-8 sequence is rounded
+		downwards to the beginning of that sequence.
 
 		Returns -1 if the arguments are invalid or if there are less
 		than {idx} bytes in {string}. If there are exactly {idx} bytes