diff --git a/src/go/token/position.go b/src/go/token/position.go
index 33751779a3..7306083b0d 100644
--- a/src/go/token/position.go
+++ b/src/go/token/position.go
@@ -164,6 +164,7 @@ func (f *File) MergeLine(line int) {
 // Each line offset must be larger than the offset for the previous line
 // and smaller than the file size; otherwise SetLines fails and returns
 // false.
+// Callers must not mutate the provided slice after SetLines returns.
 //
 func (f *File) SetLines(lines []int) bool {
 	// verify validity of lines table