diff --git a/trezor_agent/gpg/agent.py b/trezor_agent/gpg/agent.py
index dd7ca02..ca23391 100644
--- a/trezor_agent/gpg/agent.py
+++ b/trezor_agent/gpg/agent.py
@@ -66,6 +66,7 @@ def open_connection(keygrip_bytes):
 def pksign(keygrip, digest, algo):
     """Sign a message digest using a private EC key."""
     assert algo == b'8', 'Unsupported hash algorithm ID {}'.format(algo)
+    assert len(digest) == 64  # SHA-256 bits (in hex digits)
     keygrip_bytes = binascii.unhexlify(keygrip)
     with open_connection(keygrip_bytes) as conn:
         r, s = conn.sign(binascii.unhexlify(digest))