diff --git a/openpgp/packet/opaque.go b/openpgp/packet/opaque.go
index f2e00b9..456d807 100644
--- a/openpgp/packet/opaque.go
+++ b/openpgp/packet/opaque.go
@@ -6,9 +6,10 @@ package packet
 
 import (
 	"bytes"
-	"golang.org/x/crypto/openpgp/errors"
 	"io"
 	"io/ioutil"
+
+	"golang.org/x/crypto/openpgp/errors"
 )
 
 // OpaquePacket represents an OpenPGP packet as raw, unparsed data. This is
@@ -138,7 +139,7 @@ func nextSubpacket(contents []byte) (subHeaderLen int, subPacket *OpaqueSubpacke
 			uint32(contents[4])
 		contents = contents[5:]
 	}
-	if subLen > uint32(len(contents)) {
+	if subLen > uint32(len(contents)) || subLen == 0 {
 		goto Truncated
 	}
 	subPacket.SubType = contents[0]
diff --git a/openpgp/read_test.go b/openpgp/read_test.go
index 03bf5d8..0f08313 100644
--- a/openpgp/read_test.go
+++ b/openpgp/read_test.go
@@ -8,11 +8,12 @@ import (
 	"bytes"
 	_ "crypto/sha512"
 	"encoding/hex"
-	"golang.org/x/crypto/openpgp/errors"
 	"io"
 	"io/ioutil"
 	"strings"
 	"testing"
+
+	"golang.org/x/crypto/openpgp/errors"
 )
 
 func readerFromHex(s string) io.Reader {
@@ -368,6 +369,29 @@ func TestNoArmoredData(t *testing.T) {
 	}
 }
 
+func TestIssue11503(t *testing.T) {
+	data := "8c040402000aa430aa8228b9248b01fc899a91197130303030"
+
+	buf, err := hex.DecodeString(data)
+	if err != nil {
+		t.Errorf("hex.DecodeSting(): %v", err)
+	}
+
+	kr, err := ReadKeyRing(new(bytes.Buffer))
+	if err != nil {
+		t.Errorf("ReadKeyring(): %v", err)
+	}
+
+	_, err = ReadMessage(bytes.NewBuffer(buf), kr,
+		func([]Key, bool) ([]byte, error) {
+			return []byte("insecure"), nil
+		}, nil)
+
+	if err == nil {
+		t.Errorf("ReadMessage(): Unexpected nil error")
+	}
+}
+
 const testKey1KeyId = 0xA34D7E18C20C31BB
 const testKey3KeyId = 0x338934250CCC0360