Uncaught (in promise) Error: Expected a value of type 'String', but got one of type 'List<String>'

Question

My code:

          var postdata = {
            "id": "6DFA5E06-3B1C-4507-B831-2D84B4137011", 
            "subject": "Test", 
            "message": "Test", 
            "recipients": ["6CA682DD-BC13-40FB-BB31-1971553DE8F6", "763661B9-0378-4F81-8A98-6B5962752D4A"]
          },
          res = await http.post(
              Uri.parse(AuthService().baseUrl + uri),
              body: postdata, 
              headers: {
                "Accept": "application/json",
                "Content-Type": "application/x-www-form-urlencoded",
                'Authorization': 'Bearer ' + value["token"],
              },
          );

Got this error: Uncaught (in promise) Error: Expected a value of type 'String', but got one of type 'List'