diff -r bd0d13d10b8e -r 84c4e56b38d6 modbus/modbus.py
--- a/modbus/modbus.py	Thu Jun 07 16:53:21 2018 +0300
+++ b/modbus/modbus.py	Thu Jun 07 16:58:13 2018 +0300
@@ -598,10 +598,11 @@
             for j in range(i + 1, len(IPServer_port_numbers)):
                 if IPServer_port_numbers[i][1] == IPServer_port_numbers[j][1]:
                     self.GetCTRoot().logger.write_warning(
-                        _("Error: Modbus/IP Servers %s.x and %s.x use the same port number %s.\n") % (
-                            _lt_to_str(IPServer_port_numbers[i][0]),
-                            _lt_to_str(IPServer_port_numbers[j][0]),
-                            IPServer_port_numbers[j][1]))
+                        _("Error: Modbus/IP Servers %{a1}.x and %{a2}.x use the same port number {a3}.\n").\
+                        format(
+                            a1=_lt_to_str(IPServer_port_numbers[i][0]),
+                            a2=_lt_to_str(IPServer_port_numbers[j][0]),
+                            a3=IPServer_port_numbers[j][1]))
                     raise Exception
                     # TODO: return an error code instead of raising an
                     # exception