diff --git a/ecdsa.go b/ecdsa.go
index 4531d4e..96c317f 100644
--- a/ecdsa.go
+++ b/ecdsa.go
@@ -9,6 +9,7 @@ import (
   "encoding/asn1"
   "encoding/pem"
   "errors"
+  "io"
   "math/big"
 )
 
@@ -72,11 +73,11 @@ func (pr EcdsaPrivateKey) PrivateKey() crypto.PrivateKey {
 
 // This function implements the Pemmer interface to marshal the private key
 // into a pem block.
-func (pr EcdsaPrivateKey) MarshalPem() (marshalledPemBlock, error) {
+func (pr EcdsaPrivateKey) MarshalPem() (io.WriterTo, error) {
   asn1, err := x509.MarshalECPrivateKey(pr.private_key)
   if err != nil { return nil, err }
   pem_block := pem.Block{Type: PemLabelEcdsa, Bytes: asn1}
-  return pem.EncodeToMemory(&pem_block), nil
+  return marshalledPemBlock(pem.EncodeToMemory(&pem_block)), nil
 }
 
 // This functoin loads an ecdsa public key from the asn.1 representation.
@@ -91,11 +92,11 @@ func LoadPublicKeyEcdsa(raw []byte) (*EcdsaPublicKey, error) {
 
 // This function implements the Pemmer interface to marshal the public key into
 // a pem block.
-func (pu *EcdsaPublicKey) MarshalPem() (marshalledPemBlock, error) {
+func (pu *EcdsaPublicKey) MarshalPem() (io.WriterTo, error) {
   asn1, err := x509.MarshalPKIXPublicKey(pu.public_key)
   if err != nil { return nil, err }
   pem_block := pem.Block{Type: PemLabelPublic, Bytes: asn1}
-  return pem.EncodeToMemory(&pem_block), nil
+  return marshalledPemBlock(pem.EncodeToMemory(&pem_block)), nil
 }
 
 // This function verifies a message using the public key, signature and hash
diff --git a/pem_marshal.go b/pem_marshal.go
index 14b3bd4..54ce1e0 100644
--- a/pem_marshal.go
+++ b/pem_marshal.go
@@ -10,6 +10,7 @@ type (
 
 // This function writes the marshalled pem block to a writer and returns the
 // number of written bytes and eventual errors.
-func (b marshalledPemBlock) WriteTo(stream io.Writer) (int, error) {
-  return stream.Write(b)
+func (b marshalledPemBlock) WriteTo(stream io.Writer) (int64, error) {
+  numBytes, err := stream.Write(b)
+  return int64(numBytes), err
 }
diff --git a/rsa.go b/rsa.go
index ac649b7..bcdfe6e 100644
--- a/rsa.go
+++ b/rsa.go
@@ -7,6 +7,7 @@ import (
   "crypto/x509"
   "encoding/pem"
   "errors"
+  "io"
 )
 
 const (
@@ -50,10 +51,10 @@ func (pr RsaPrivateKey) PrivateKey() crypto.PrivateKey {
   return pr.private_key
 }
 
-func (pr RsaPrivateKey) MarshalPem() (marshalledPemBlock, error) {
+func (pr RsaPrivateKey) MarshalPem() (io.WriterTo, error) {
   asn1 := x509.MarshalPKCS1PrivateKey(pr.private_key)
   pem_block := pem.Block{Type: PemLabelRsa, Bytes: asn1}
-  return pem.EncodeToMemory(&pem_block), nil
+  return marshalledPemBlock(pem.EncodeToMemory(&pem_block)), nil
 }
 
 // restore a rsa public key
@@ -62,11 +63,11 @@ func LoadPublicKeyRsa(raw []byte) (*RsaPublicKey, error) {
 }
 
 // marshal a rsa public key into pem format
-func (pu *RsaPublicKey) MarshalPem() (marshalledPemBlock, error) {
+func (pu *RsaPublicKey) MarshalPem() (io.WriterTo, error) {
   asn1, err := x509.MarshalPKIXPublicKey(pu.public_key)
   if err != nil { return nil, err }
   pem_block := pem.Block{Type: PemLabelPublic, Bytes: asn1}
-  return pem.EncodeToMemory(&pem_block), nil
+  return marshalledPemBlock(pem.EncodeToMemory(&pem_block)), nil
 }
 
 // verify a message with a signature using the public key
diff --git a/types.go b/types.go
index 43150f9..08c42d8 100644
--- a/types.go
+++ b/types.go
@@ -16,6 +16,7 @@ package pki
 
 import (
   "crypto"
+  "io"
 )
 
 // This label is used as the type in the pem encoding of public keys.
@@ -47,6 +48,6 @@ type (
   // in the pem format. The result can then be written to any structure
   // implementing the io.Writer interface.
   Pemmer interface {
-    MarshalPem() (marshalledPemBlock, error)
+    MarshalPem() (io.WriterTo, error)
   }
 )