diff --git a/vector/util.go b/vector/util.go
index 0465181a8..10c1f3990 100644
--- a/vector/util.go
+++ b/vector/util.go
@@ -55,6 +55,8 @@ func drawVerticesForUtil(dst *ebiten.Image, vs []ebiten.Vertex, is []uint16, clr
 }
 
 // StrokeLine strokes a line (x0, y0)-(x1, y1) with the specified width and color.
+//
+// clr has be to be a solid (non-transparent) color.
 func StrokeLine(dst *ebiten.Image, x0, y0, x1, y1 float32, strokeWidth float32, clr color.Color, antialias bool) {
 	var path Path
 	path.MoveTo(x0, y0)