diff --git a/ipv4/control_windows.go b/ipv4/control_windows.go
index ce55c66..82c6306 100644
--- a/ipv4/control_windows.go
+++ b/ipv4/control_windows.go
@@ -4,13 +4,9 @@
 
 package ipv4
 
-import (
-	"syscall"
-
-	"golang.org/x/net/internal/socket"
-)
+import "golang.org/x/net/internal/socket"
 
 func setControlMessage(c *socket.Conn, opt *rawOpt, cf ControlFlags, on bool) error {
 	// TODO(mikio): implement this
-	return syscall.EWINDOWS
+	return errNotImplemented
 }
diff --git a/ipv4/header.go b/ipv4/header.go
index a8c8f7a..701bd4b 100644
--- a/ipv4/header.go
+++ b/ipv4/header.go
@@ -57,7 +57,7 @@
 // This may differ from the wire format, depending on the system.
 func (h *Header) Marshal() ([]byte, error) {
 	if h == nil {
-		return nil, errInvalidConn
+		return nil, errNilHeader
 	}
 	if h.Len < HeaderLen {
 		return nil, errHeaderTooShort
@@ -107,12 +107,15 @@
 // local system.
 // This may differ from the wire format, depending on the system.
 func (h *Header) Parse(b []byte) error {
-	if h == nil || len(b) < HeaderLen {
+	if h == nil || b == nil {
+		return errNilHeader
+	}
+	if len(b) < HeaderLen {
 		return errHeaderTooShort
 	}
 	hdrlen := int(b[0]&0x0f) << 2
-	if hdrlen > len(b) {
-		return errBufferTooShort
+	if len(b) < hdrlen {
+		return errExtHeaderTooShort
 	}
 	h.Version = int(b[0] >> 4)
 	h.Len = hdrlen
diff --git a/ipv4/header_test.go b/ipv4/header_test.go
index a246aee..2211605 100644
--- a/ipv4/header_test.go
+++ b/ipv4/header_test.go
@@ -157,10 +157,21 @@
 }
 
 func TestMarshalHeader(t *testing.T) {
+	for i, tt := range []struct {
+		h   *Header
+		err error
+	}{
+		{nil, errNilHeader},
+		{&Header{Len: HeaderLen - 1}, errHeaderTooShort},
+	} {
+		if _, err := tt.h.Marshal(); err != tt.err {
+			t.Errorf("#%d: got %v; want %v", i, err, tt.err)
+		}
+	}
+
 	if socket.NativeEndian != binary.LittleEndian {
 		t.Skip("no test for non-little endian machine yet")
 	}
-
 	for _, tt := range headerLittleEndianTests {
 		b, err := tt.Header.Marshal()
 		if err != nil {
@@ -189,10 +200,30 @@
 }
 
 func TestParseHeader(t *testing.T) {
+	for i, tt := range []struct {
+		h   *Header
+		wh  []byte
+		err error
+	}{
+		{nil, nil, errNilHeader},
+		{&Header{}, nil, errNilHeader},
+		{&Header{}, make([]byte, HeaderLen-1), errHeaderTooShort},
+		{&Header{}, []byte{
+			0x46, 0x00, 0x00, 0x00,
+			0x00, 0x00, 0x00, 0x00,
+			0x00, 0x00, 0x00, 0x00,
+			0x00, 0x00, 0x00, 0x00,
+			0x00, 0x00, 0x00, 0x00,
+		}, errExtHeaderTooShort},
+	} {
+		if err := tt.h.Parse(tt.wh); err != tt.err {
+			t.Fatalf("#%d: got %v; want %v", i, err, tt.err)
+		}
+	}
+
 	if socket.NativeEndian != binary.LittleEndian {
 		t.Skip("no test for big endian machine yet")
 	}
-
 	for _, tt := range headerLittleEndianTests {
 		var wh []byte
 		switch runtime.GOOS {
diff --git a/ipv4/helper.go b/ipv4/helper.go
index 8d8ff98..ffedd4e 100644
--- a/ipv4/helper.go
+++ b/ipv4/helper.go
@@ -7,18 +7,21 @@
 import (
 	"errors"
 	"net"
+	"runtime"
 )
 
 var (
 	errInvalidConn              = errors.New("invalid connection")
 	errMissingAddress           = errors.New("missing address")
 	errMissingHeader            = errors.New("missing header")
+	errNilHeader                = errors.New("nil header")
 	errHeaderTooShort           = errors.New("header too short")
-	errBufferTooShort           = errors.New("buffer too short")
+	errExtHeaderTooShort        = errors.New("extension header too short")
 	errInvalidConnType          = errors.New("invalid conn type")
 	errOpNoSupport              = errors.New("operation not supported")
 	errNoSuchInterface          = errors.New("no such interface")
 	errNoSuchMulticastInterface = errors.New("no such multicast interface")
+	errNotImplemented           = errors.New("not implemented on " + runtime.GOOS + "/" + runtime.GOARCH)
 
 	// See http://www.freebsd.org/doc/en/books/porters-handbook/freebsd-versions.html.
 	freebsdVersion uint32