diff --git a/src/allmydata/test/matchers.py b/src/allmydata/test/matchers.py
index eaf7d13a5..2d5e74ffd 100644
--- a/src/allmydata/test/matchers.py
+++ b/src/allmydata/test/matchers.py
@@ -51,7 +51,7 @@ class MatchesNodePublicKey(object):
         :return Mismatch: If the keys don't match.
         """
         config = read_config(self.basedir, u"tub.port")
-        privkey_bytes = config.get_private_config("node.privkey")
+        privkey_bytes = config.get_private_config("node.privkey").encode("utf-8")
         private_key = ed25519.signing_keypair_from_string(privkey_bytes)[0]
         signature = ed25519.sign_data(private_key, b"")
         other_public_key = ed25519.verifying_key_from_signing_key(other)