diff --git a/README.md b/README.md
index 52c1fd4a..3bfcb30a 100644
--- a/README.md
+++ b/README.md
@@ -17,6 +17,7 @@ CAD program, and create G-Code for Isolation routing.
 - in NCC Tool I've added a warning so the user is warned that the NCC margin has to have a value of at least the tool diameter that is doing an iso_op job in the Tool Table
 - modified the Drillcncjob and Cncjob Tcl commands to be allowed to work without the 'dwell' and 'toolchange' arguments. If 'dwelltime' argument is present it will be assumed that the 'dwell' is True and the same for 'toolchangez' parameter, if present then 'toolchange' will be assumed to be True, else False
 - modified the extracut and multidepth parameters in Cncjob Tcl command like for dwell and toolchange
+- added ability for Tcl commands to have optional arguments with None value (meaning missing value). This case should be treated for each Tcl command in execute() method
 
 30.12.2019
 
diff --git a/tclCommands/TclCommand.py b/tclCommands/TclCommand.py
index 294e0197..719b3c71 100644
--- a/tclCommands/TclCommand.py
+++ b/tclCommands/TclCommand.py
@@ -177,19 +177,26 @@ class TclCommand(object):
         arguments = []
         n = len(args)
 
-        name = None
+        option_name = None
+
         for i in range(n):
             match = re.search(r'^-([a-zA-Z].*)', args[i])
             if match:
-                assert name is None
-                name = match.group(1)
+                # assert option_name is None
+                if option_name is not None:
+                    options[option_name] = None
+
+                option_name = match.group(1)
                 continue
 
-            if name is None:
+            if option_name is None:
                 arguments.append(args[i])
             else:
-                options[name] = args[i]
-                name = None
+                options[option_name] = args[i]
+                option_name = None
+
+        if option_name is not None:
+            options[option_name] = None
 
         return arguments, options
 
@@ -211,6 +218,7 @@ class TclCommand(object):
         for argument in arguments:
             if len(self.arg_names) > idx:
                 key, arg_type = arg_names_items[idx]
+
                 try:
                     named_args[key] = arg_type(argument)
                 except Exception as e:
@@ -226,7 +234,12 @@ class TclCommand(object):
                 self.raise_tcl_error('Unknown parameter: %s' % key)
             try:
                 if key != 'timeout':
-                    named_args[key] = self.option_types[key](options[key])
+                    # None options are allowed; if None then the defaults are used
+                    # - must be implemented in the Tcl commands
+                    if options[key] is not None:
+                        named_args[key] = self.option_types[key](options[key])
+                    else:
+                        named_args[key] = options[key]
                 else:
                     named_args[key] = int(options[key])
             except Exception as e: