Multilov on Regular Field

This single LoV works without problem -

lov UseridRef with ReferenceFndUserSelector {
      search = Identity, Description;
      description = UseridRef.Description;
      contactwidget {
         enabled = [true];
         source = Person;
      }
   }

But the attempt to convert it to mulitple LoV, boils up in error -
 

   multilov UseridRef with ReferenceFndUserSelector using FndUserSet(Person) {
      validate command {
         execute {
         }

      }
   }

with detail -
 

...

Error stack:

Bad metadata error:

The list: ReferenceFndUserSelector does not exist in dictionary!

Error

 ...    

Any clues please? The code was copied from an already working location,

Thank you.