diff --git a/typesetter.go b/typesetter.go
index f5e38d2..3ddcf5b 100644
--- a/typesetter.go
+++ b/typesetter.go
@@ -89,7 +89,8 @@ type TypeSetter struct {
 	layoutBoundsSpace fixed.Rectangle26_6
 }
 
-// AtPosition returns the index of the rune at the specified position.
+// AtPosition returns the index of the rune at the specified position. The
+// returned index may be greater than the length of runes in the TypeSetter.
 func (this *TypeSetter) AtPosition (position fixed.Point26_6) int {
 	metrics := this.face.Metrics()
 	lastValidIndex := 0