diff --git a/src/image/draw/bench_test.go b/src/image/draw/bench_test.go
index 2b7c6d6ac4..55d25b8596 100644
--- a/src/image/draw/bench_test.go
+++ b/src/image/draw/bench_test.go
@@ -190,8 +190,7 @@ func bench(b *testing.B, dcm, scm, mcm color.Model, op Op) {
 	}
 }
 
-// The BenchmarkFoo and BenchmarkFooN functions exercise a drawFoo fast-path
-// function in draw.go.
+// The BenchmarkFoo functions exercise a drawFoo fast-path function in draw.go.
 
 func BenchmarkFillOver(b *testing.B) {
 	bench(b, color.RGBAModel, nil, nil, Over)
@@ -233,12 +232,20 @@ func BenchmarkGlyphOver(b *testing.B) {
 	bench(b, color.RGBAModel, nil, color.AlphaModel, Over)
 }
 
-func BenchmarkRGBA1(b *testing.B) {
-	bench(b, color.RGBAModel, color.RGBA64Model, nil, Src)
+func BenchmarkRGBAMaskOver(b *testing.B) {
+	bench(b, color.RGBAModel, color.RGBAModel, color.AlphaModel, Over)
 }
 
-func BenchmarkRGBA2(b *testing.B) {
-	bench(b, color.RGBAModel, color.RGBAModel, color.AlphaModel, Over)
+func BenchmarkGrayMaskOver(b *testing.B) {
+	bench(b, color.RGBAModel, color.GrayModel, color.AlphaModel, Over)
+}
+
+func BenchmarkRGBA64ImageMaskOver(b *testing.B) {
+	bench(b, color.RGBAModel, color.RGBA64Model, color.AlphaModel, Over)
+}
+
+func BenchmarkRGBA(b *testing.B) {
+	bench(b, color.RGBAModel, color.RGBA64Model, nil, Src)
 }
 
 func BenchmarkPalettedFill(b *testing.B) {
@@ -266,11 +273,3 @@ func BenchmarkGenericSrc(b *testing.B) {
 func BenchmarkGenericMaskSrc(b *testing.B) {
 	bench(b, color.RGBA64Model, color.RGBA64Model, color.AlphaModel, Src)
 }
-
-func BenchmarkRGBA64Over(b *testing.B) {
-	bench(b, color.RGBAModel, color.RGBA64Model, color.AlphaModel, Over)
-}
-
-func BenchmarkGrayOver(b *testing.B) {
-	bench(b, color.RGBAModel, color.GrayModel, color.AlphaModel, Over)
-}
diff --git a/src/image/draw/draw.go b/src/image/draw/draw.go
index d3c5b29097..7dd18dfdb5 100644
--- a/src/image/draw/draw.go
+++ b/src/image/draw/draw.go
@@ -166,6 +166,8 @@ func DrawMask(dst Image, r image.Rectangle, src image.Image, sp image.Point, mas
 				case *image.Gray:
 					drawGrayMaskOver(dst0, r, src0, sp, mask0, mp)
 					return
+				// Case order matters. The next case (image.RGBA64Image) is an
+				// interface type that the concrete types above also implement.
 				case image.RGBA64Image:
 					drawRGBA64ImageMaskOver(dst0, r, src0, sp, mask0, mp)
 					return
diff --git a/src/image/draw/draw_test.go b/src/image/draw/draw_test.go
index 75a2896631..77f1c5c2c2 100644
--- a/src/image/draw/draw_test.go
+++ b/src/image/draw/draw_test.go
@@ -380,7 +380,7 @@ var drawTests = []drawTest{
 	{"cmykAlphaSrc", vgradMagenta(), fillAlpha(192), Src, color.RGBA{145, 67, 145, 192}},
 	{"cmykNil", vgradMagenta(), nil, Over, color.RGBA{192, 89, 192, 255}},
 	{"cmykNilSrc", vgradMagenta(), nil, Src, color.RGBA{192, 89, 192, 255}},
-	// Variable mask and variable source.
+	// Variable mask and uniform source.
 	// At (x, y) == (8, 8):
 	// The destination pixel is {136, 0, 0, 255}.
 	// The source pixel is {0, 0, 255, 255}.
@@ -397,9 +397,14 @@ var drawTests = []drawTest{
 		Over, color.RGBA{81, 0, 102, 255}},
 	{"genericSrcSlowest", fillBlue(255), convertToSlowestRGBA(vgradAlpha(192)),
 		Src, color.RGBA{0, 0, 102, 102}},
-	// The source pixel is {0, 48, 0, 90}.
-	{"rgbaVariableMaskOver", vgradGreen(255), vgradAlpha(192), Over, color.RGBA{81, 54, 0, 255}},
-	// The source pixel is {136} in Gray-space, which is {136, 136, 136, 255} in RGBA-space.
+	// Variable mask and variable source.
+	// At (x, y) == (8, 8):
+	// The destination pixel is {136, 0, 0, 255}.
+	// The source pixel is:
+	//   - {0, 48, 0, 90}.
+	//   - {136} in Gray-space, which is {136, 136, 136, 255} in RGBA-space.
+	// The mask pixel's alpha is 102, or 40%.
+	{"rgbaVariableMaskOver", vgradGreen(90), vgradAlpha(192), Over, color.RGBA{117, 19, 0, 255}},
 	{"grayVariableMaskOver", vgradGray(), vgradAlpha(192), Over, color.RGBA{136, 54, 54, 255}},
 }