diff --git a/sshagent/formats.py b/sshagent/formats.py
index df57cc3..d1592f0 100644
--- a/sshagent/formats.py
+++ b/sshagent/formats.py
@@ -76,7 +76,7 @@ def export_public_key(pubkey, label):
                                               hashfunc=hashfunc)
     key_type = 'ecdsa-sha2-nistp256'
     curve_name = 'nistp256'
-    parts = [key_type, curve_name, '\x04' + vk.to_string()]
+    parts = [key_type, curve_name, DER_OCTET_STRING + vk.to_string()]
     blob = ''.join([util.frame(p) for p in parts])
     log.debug('fingerprint: %s', fingerprint(blob))
     b64 = base64.b64encode(blob)