diff --git a/trezorctl b/trezorctl
index 12098cd..df8a4f2 100755
--- a/trezorctl
+++ b/trezorctl
@@ -591,7 +591,7 @@ def main():
             print(res)
     except CallException as e:
         status, message = e.args
-        sys.stderr.write('failure: {message}\n'.format(message=message))
+        sys.stderr.write(b'failure: {message}\n'.format(message=message))
         exit(status)
 
 
diff --git a/trezorlib/client.py b/trezorlib/client.py
index 83b328e..d54436e 100644
--- a/trezorlib/client.py
+++ b/trezorlib/client.py
@@ -90,7 +90,7 @@ def pprint(msg):
         return "<%s> (%d bytes):\n%s" % (msg_class, msg_size, msg)
 
 def log(msg):
-    sys.stderr.write("%s\n" % msg)
+    sys.stderr.write(b'%s\n' % msg)
     sys.stderr.flush()
 
 class CallException(Exception):