diff --git a/mod.go b/mod.go
index 7126a36..f26d627 100644
--- a/mod.go
+++ b/mod.go
@@ -330,8 +330,8 @@ func Reciprocal(m *Int) (mu [5]uint64) {
 
 // reduce4 computes the least non-negative residue of x modulo m
 //
-// requires a four-word modulus (m[3] > 1) and its inverse (mu)
-func reduce4(x [8]uint64, m *Int, mu [5]uint64) (z Int) {
+// requires a four-word modulus (m[3] != 0) and its inverse (mu)
+func (z *Int) reduce4(x *[8]uint64, m *Int, mu *[5]uint64) *Int {
 
 	// NB: Most variable names in the comments match the pseudocode for
 	// 	Barrett reduction in the Handbook of Applied Cryptography.
diff --git a/uint256.go b/uint256.go
index 1f4de33..904bf4d 100644
--- a/uint256.go
+++ b/uint256.go
@@ -679,8 +679,7 @@ func (z *Int) MulModWithReciprocal(x, y, m *Int, mu *[5]uint64) *Int {
 	umul(x, y, &p)
 
 	if m[3] != 0 {
-		r := reduce4(p, m, *mu)
-		return z.Set(&r)
+		return z.reduce4(&p, m, mu)
 	}
 
 	var (
@@ -713,8 +712,7 @@ func (z *Int) MulMod(x, y, m *Int) *Int {
 
 	if m[3] != 0 {
 		mu := Reciprocal(m)
-		r := reduce4(p, m, mu)
-		return z.Set(&r)
+		return z.reduce4(&p, m, &mu)
 	}
 
 	var (