diff --git a/trezor_agent/gpg/tests/test_decode.py b/trezor_agent/gpg/tests/test_decode.py
index 88b7ae4..92c9b8c 100644
--- a/trezor_agent/gpg/tests/test_decode.py
+++ b/trezor_agent/gpg/tests/test_decode.py
@@ -43,3 +43,16 @@ def test_gpg_files(public_key_path):  # pylint: disable=redefined-outer-name
     with open(public_key_path, 'rb') as f:
         packets = list(decode.parse_packets(f))
         assert len(packets) > 0
+
+
+def test_has_custom_subpacket():
+    sig = {'unhashed_subpackets': []}
+    assert not decode.has_custom_subpacket(sig)
+
+    custom_markers = [
+        protocol.CUSTOM_SUBPACKET,
+        protocol.subpacket(10, protocol.CUSTOM_KEY_LABEL),
+    ]
+    for marker in custom_markers:
+        sig = {'unhashed_subpackets': [marker]}
+        assert decode.has_custom_subpacket(sig)