Using Enumeration values in a DSL

Hello,

In my language, I have defined a Enumeration type named "Operator", with values such as "AND", "OR" etc...
This type is used as a property in a Concept.
The editor of this concept refers to it simply by:
[- { name } { operator } ...

The build of the language succeed, but when I try to instanciate it with a test model, I can't fill the cell corresponding to this property.
The auto-completion propose all the values of the enum type I declared, but when I select one nothing changes in my AST (and my model is however valide).
I have an hover hint : "Error: Incorrect child role used: LinkDeclaration with role "operator" was not found in parent node's concept"

Is there an example of a language using enumeration in /MPSSamples.3.0/ ?
I am sure that the answer is simple, but I can't figure out what I'm not doing right.

Thank you for your help !