diff --git a/codec.go b/codec.go
index 83b42a0e..c68ec375 100644
--- a/codec.go
+++ b/codec.go
@@ -195,6 +195,10 @@ func MustUnmarshalRoot(data []byte) Ptr {
 	return p
 }
 
+var (
+	errTooManySegments = errors.New("message has too many segments")
+)
+
 // An Encoder represents a framer for serializing a particular Cap'n
 // Proto stream.
 type Encoder struct {
@@ -220,6 +224,9 @@ func (e *Encoder) Encode(m *Message) error {
 	if nsegs == 0 {
 		return errors.New("encode: message has no segments")
 	}
+	if nsegs > 1<<32 {
+		return exc.WrapError("encode", errTooManySegments)
+	}
 	e.bufs = append(e.bufs[:0], nil) // first element is placeholder for header
 	maxSeg := SegmentID(nsegs - 1)
 	hdrSize := streamHeaderSize(maxSeg)
diff --git a/codec_test.go b/codec_test.go
index 1e194123..9a66c9d8 100644
--- a/codec_test.go
+++ b/codec_test.go
@@ -2,8 +2,11 @@ package capnp
 
 import (
 	"bytes"
+	"errors"
 	"io"
 	"testing"
+
+	"github.com/stretchr/testify/require"
 )
 
 func TestEncoder(t *testing.T) {
@@ -72,6 +75,40 @@ func TestDecoder(t *testing.T) {
 	}
 }
 
+type tooManySegsArena struct {
+	data []byte
+}
+
+func (t *tooManySegsArena) NumSegments() int64 { return 1<<32 + 1 }
+
+func (t *tooManySegsArena) Data(id SegmentID) ([]byte, error) {
+	return nil, errors.New("no data")
+}
+
+func (t *tooManySegsArena) Allocate(minsz Size, segs map[SegmentID]*Segment) (SegmentID, []byte, error) {
+	return 0, nil, errors.New("cannot allocate")
+}
+
+func (t *tooManySegsArena) Release() {}
+
+// TestEncoderTooManySegments verifies attempting to encode an arena that has
+// more segments than possible.
+func TestEncoderTooManySegments(t *testing.T) {
+	t.Parallel()
+	zeroWord := []byte{0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00}
+	arena := &tooManySegsArena{data: zeroWord}
+
+	// Setup via field because NewMessage checks arena has > 1 segments.
+	var msg Message
+	msg.Arena = arena
+	var buf bytes.Buffer
+	enc := NewEncoder(&buf)
+	err := enc.Encode(&msg)
+
+	// Encoding should error with a specific error.
+	require.ErrorIs(t, err, errTooManySegments)
+}
+
 func TestDecoder_MaxMessageSize(t *testing.T) {
 	t.Parallel()