diff --git a/mfer/deserialize.go b/mfer/deserialize.go
index 6da64c6..60c93b0 100644
--- a/mfer/deserialize.go
+++ b/mfer/deserialize.go
@@ -278,6 +278,10 @@ func (m *manifest) deserializeInner() error {
 		return fmt.Errorf("deserialize: unmarshal inner: %w", err)
 	}
 
+	if m.pbInner.GetVersion() != MFFile_VERSION_ONE {
+		return errUnknownVersion
+	}
+
 	// Validate inner UUID
 	err = validateUUID(m.pbInner.GetUuid())
 	if err != nil {
diff --git a/mfer/deserialize_path_test.go b/mfer/deserialize_path_test.go
index 8572901..aec2e92 100644
--- a/mfer/deserialize_path_test.go
+++ b/mfer/deserialize_path_test.go
@@ -151,7 +151,9 @@ func TestDeserializeRefusesEntriesThatDecodeTooLarge(t *testing.T) {
 			entry = protowire.AppendBytes(entry, nil)
 
 			id := uuid.NewV4()
-			inner := protowire.AppendTag(nil, 102, protowire.BytesType) // MFFile.uuid
+			inner := protowire.AppendTag(nil, 100, protowire.VarintType) // MFFile.version
+			inner = protowire.AppendVarint(inner, uint64(MFFile_VERSION_ONE))
+			inner = protowire.AppendTag(inner, 102, protowire.BytesType) // MFFile.uuid
 			inner = protowire.AppendBytes(inner, id[:])
 
 			for range 1000 {
@@ -182,7 +184,9 @@ func TestDeserializeDropsUnknownFields(t *testing.T) {
 	entry = append(entry, unknown...)
 
 	id := uuid.NewV4()
-	inner := protowire.AppendTag(nil, 101, protowire.BytesType) // MFFile.files
+	inner := protowire.AppendTag(nil, 100, protowire.VarintType) // MFFile.version
+	inner = protowire.AppendVarint(inner, uint64(MFFile_VERSION_ONE))
+	inner = protowire.AppendTag(inner, 101, protowire.BytesType) // MFFile.files
 	inner = protowire.AppendBytes(inner, entry)
 	inner = protowire.AppendTag(inner, 102, protowire.BytesType) // MFFile.uuid
 	inner = protowire.AppendBytes(inner, id[:])
diff --git a/mfer/deserialize_test.go b/mfer/deserialize_test.go
index 0a8f272..878a6a8 100644
--- a/mfer/deserialize_test.go
+++ b/mfer/deserialize_test.go
@@ -4,11 +4,32 @@ package mfer
 import (
 	"bytes"
 	"testing"
+	"uuid"
 
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
+	"google.golang.org/protobuf/proto"
 )
 
+// An inner message whose version is not VERSION_ONE, whether version 0 or a
+// later one, is refused with the same error as an outer message's.
+func TestDeserializeRefusesUnknownInnerVersion(t *testing.T) {
+	t.Parallel()
+
+	for _, version := range []MFFile_Version{MFFile_VERSION_NONE, MFFile_VERSION_ONE + 1} {
+		t.Run(version.String(), func(t *testing.T) {
+			t.Parallel()
+
+			id := uuid.NewV4()
+			inner, err := proto.Marshal(&MFFile{Version: version, Uuid: id[:]})
+			require.NoError(t, err)
+
+			_, err = NewManifestFromReader(bytes.NewReader(wrapInner(t, id, inner)))
+			require.ErrorIs(t, err, errUnknownVersion)
+		})
+	}
+}
+
 // TestReadAtMost gives readAtMost exactly its maximum, which it must
 // return whole, and twice its maximum, which it must refuse after reading
 // one byte past the maximum, and no more. NewManifestFromReader reads