diff --git a/trezor_agent/gpg/protocol.py b/trezor_agent/gpg/protocol.py
index e0f0709..b4d4e01 100644
--- a/trezor_agent/gpg/protocol.py
+++ b/trezor_agent/gpg/protocol.py
@@ -47,9 +47,22 @@ def subpacket_byte(subpacket_type, value):
     return subpacket(subpacket_type, '>B', value)
 
 
+def subpacket_prefix_len(item):
+    """Prefix subpacket length according to RFC 4880 section-5.2.3.1."""
+    n = len(item)
+    if n >= 8384:
+        prefix = b'\xFF' + struct.pack('>L', n)
+    elif n >= 192:
+        n = n - 192
+        prefix = struct.pack('BB', (n // 256) + 192, n % 256)
+    else:
+        prefix = struct.pack('B', n)
+    return prefix + item
+
+
 def subpackets(*items):
     """Serialize several GPG subpackets."""
-    prefixed = [util.prefix_len('>B', item) for item in items]
+    prefixed = [subpacket_prefix_len(item) for item in items]
     return util.prefix_len('>H', b''.join(prefixed))
 
 
diff --git a/trezor_agent/gpg/tests/test_decode.py b/trezor_agent/gpg/tests/test_decode.py
index 00d5eec..5480f80 100644
--- a/trezor_agent/gpg/tests/test_decode.py
+++ b/trezor_agent/gpg/tests/test_decode.py
@@ -5,7 +5,7 @@ import os
 
 import pytest
 
-from .. import decode
+from .. import decode, protocol
 from ... import util
 
 
@@ -14,6 +14,15 @@ def test_subpackets():
     assert decode.parse_subpackets(util.Reader(s)) == [b'\xAB\xCD', b'\xEF']
 
 
+def test_subpackets_prefix():
+    for n in [0, 1, 2, 4, 5, 10, 191, 192, 193,
+              255, 256, 257, 8383, 8384, 65530]:
+        item = b'?' * n  # create dummy subpacket
+        prefixed = protocol.subpackets(item)
+        result = decode.parse_subpackets(util.Reader(io.BytesIO(prefixed)))
+        assert [item] == result
+
+
 def test_mpi():
     s = io.BytesIO(b'\x00\x09\x01\x23')
     assert decode.parse_mpi(util.Reader(s)) == 0x123