I create a custom activity with with a radio button Format, when I drag and drop my activity in the designer it react as expected

which is created with the following code

 public enum FormatExport { Csv = 0, Excel = 1 }
        private FormatExport format = FormatExport.Csv;
        public FormatExport Format
        {
            get { return this.format; }
            set { this.format = value; }
        }

Here are my designer parameters

but when I try to publish the workflow using this activity I have the following Error

Any idea what I do wrong ?

if I add the enum after the class the message is more explicit