diff --git a/cid.go b/cid.go
index 99a26a6..0366080 100644
--- a/cid.go
+++ b/cid.go
@@ -145,10 +145,10 @@ func NewCidV1(codecType uint64, mhash mh.Multihash) *Cid {
 }
 
 // NewPrefixV0 returns a CIDv0 prefix with the specified multihash type.
-func NewPrefixV0(mhType uint64) Prefix {
+func NewPrefixV0() Prefix {
 	return Prefix{
-		MhType:   mhType,
-		MhLength: mh.DefaultLengths[mhType],
+		MhType:   mh.SHA2_256,
+		MhLength: mh.DefaultLengths[mh.SHA2_256],
 		Version:  0,
 		Codec:    DagProtobuf,
 	}
@@ -156,10 +156,13 @@ func NewPrefixV0(mhType uint64) Prefix {
 
 // NewPrefixV1 returns a CIDv1 prefix with the specified codec and multihash
 // type.
-func NewPrefixV1(codecType uint64, mhType uint64) Prefix {
+func NewPrefixV1(codecType uint64, mhType uint64, mhLen int) Prefix {
+	if mhLen == -1 {
+		mhLen = mh.DefaultLengths[mhType]
+	}
 	return Prefix{
 		MhType:   mhType,
-		MhLength: mh.DefaultLengths[mhType],
+		MhLength: mhLen,
 		Version:  1,
 		Codec:    codecType,
 	}
diff --git a/cid_test.go b/cid_test.go
index b7bdd89..e035b57 100644
--- a/cid_test.go
+++ b/cid_test.go
@@ -193,7 +193,7 @@ func TestNewPrefixV1(t *testing.T) {
 	data := []byte("this is some test content")
 
 	// Construct c1
-	prefix := NewPrefixV1(DagCBOR, mh.SHA2_256)
+	prefix := NewPrefixV1(DagCBOR, mh.SHA2_256, -1)
 	c1, err := prefix.Sum(data)
 	if err != nil {
 		t.Fatal(err)
@@ -222,7 +222,7 @@ func TestNewPrefixV0(t *testing.T) {
 	data := []byte("this is some test content")
 
 	// Construct c1
-	prefix := NewPrefixV0(mh.SHA2_256)
+	prefix := NewPrefixV0()
 	c1, err := prefix.Sum(data)
 	if err != nil {
 		t.Fatal(err)