diff --git a/colorm.go b/colorm.go
index dd7a749a9..12b8b43a7 100644
--- a/colorm.go
+++ b/colorm.go
@@ -60,27 +60,23 @@ func (c ColorM) Element(i, j int) float64 {
 }
 
 // Concat multiplies a color matrix with the other color matrix.
-// This returns c.
-func (c *ColorM) Concat(other ColorM) ColorM {
+func (c *ColorM) Concat(other ColorM) {
 	if !c.initialized {
 		*c = colorMI
 	}
 	result := ColorM{}
 	mul(&other, c, &result)
 	*c = result
-	return *c
 }
 
 // Add adds a color matrix with the other color matrix.
-// This returns c.
-func (c *ColorM) Add(other ColorM) ColorM {
+func (c *ColorM) Add(other ColorM) {
 	if !c.initialized {
 		*c = colorMI
 	}
 	result := ColorM{}
 	add(&other, c, &result)
 	*c = result
-	return *c
 }
 
 // SetElement sets an element at (i, j).
diff --git a/geom.go b/geom.go
index 3482c5eff..84e721445 100644
--- a/geom.go
+++ b/geom.go
@@ -53,27 +53,23 @@ func (g GeoM) Element(i, j int) float64 {
 }
 
 // Concat multiplies a geometry matrix with the other geometry matrix.
-// This returns g.
-func (g *GeoM) Concat(other GeoM) GeoM {
+func (g *GeoM) Concat(other GeoM) {
 	if !g.initialized {
 		*g = geoMI
 	}
 	result := GeoM{}
 	mul(&other, g, &result)
 	*g = result
-	return *g
 }
 
 // Add adds a geometry matrix with the other geometry matrix.
-// This returns g.
-func (g *GeoM) Add(other GeoM) GeoM {
+func (g *GeoM) Add(other GeoM) {
 	if !g.initialized {
 		*g = geoMI
 	}
 	result := GeoM{}
 	add(&other, g, &result)
 	*g = result
-	return *g
 }
 
 // SetElement sets an element at (i, j).