diff --git a/color.go b/color.go
index a70ff72..032f406 100644
--- a/color.go
+++ b/color.go
@@ -67,6 +67,9 @@ func (c NRGBA) RGBA() (r, g, b, a uint32) {
 
 // NRGBAModel converts colors to NRGBA format.
 var NRGBAModel = color.ModelFunc(func(c color.Color) color.Color {
+	if c, ok := c.(NRGBA); ok {
+		return c
+	}
 	r, g, b, a := c.RGBA()
 	if a == 0 {
 		return NRGBA{0, 0, 0, 0}