Argument with drop down menu

Hello,

I want to set an argument with a drop down menu. I can see this is done in for example Biquad Sparse V6.

I could not find documentation on any fields of the argument. For a variable, I found the following way:

value_range = [0 1];
value_list = 'val1=0 val2=1';
add_variable(M, 'my_var', 'int', 0, 'parameter', 'Set my_var.');
M.my_var.range=value_range ;
M.my_var.isRangeEditable=0;
M.my_var.isHidden=0;
M.my_var.guiInfo.controlType='droplist';
M.my_var.guiInfo.nameValueList=value_list;

But I need this for an argument, so I tried it like this:

value_list = 'enum val1=0,val2=1';

add_argument(M, 'MY_VAR', value_list , 0,  'const',  'Comment.') 

This gives me the drop down for the argument, but when I switch to val2 in the dropwdown,

I get the following error:

  ERROR: Event handler returned an error.
  Error stored in AWE_INFO.designer.lastComErr. Type:
  rethrow(AWE_INFO.designer.lastComErr)
  to examine the error in detail.
  Summary Message: This data type is not yet handled  File: target_number_to_string
  Line: 0
>> rethrow(AWE_INFO.designer.lastComErr)
Unable to resolve the name 'AWE_INFO.designer.lastComErr'.

What can I do?

Thanks