In the following case, when using assignment on an optional vector of enum values I cannot explain the behavior of the code. Why does assignment of ... = {C} compile and create a vector of size 2, while ... = {2} does not compile?

I have a fairly simple database which uses a unique id (starting at zero) to represent a table id. Each record within each table has its own record id which is unique to that table. E.g. the first record of each table will have a record id of zero.