diff --git a/greenctld b/greenctld
index bc8ad97..0141ab3 100755
--- a/greenctld
+++ b/greenctld
@@ -63,8 +63,8 @@ class GreenHeronRotor(object):
 
     def set_pos(self, az, el):
         print('==> %d,%d' % (az, el))
-        self.az_serial.write('AP1%03d\r;' % az)
-        self.el_serial.write('AP1%03d\r;' % el)
+        self.az_serial.write('AP1%03d\r;'.encode('utf-8') % az)
+        self.el_serial.write('AP1%03d\r;'.encode('utf-8') % el)
         time.sleep(0.1)
 
     def __parse_response(self, buf):