String array default throwing errors

PowerShell Universal
1 
Product: PowerShell Universal
Version: 3.3.4

Using the string array param in a script with a default results in a error of;

The argument ““UserMailbox”” does not belong to the set “UserMailbox,SharedMailbox,SchedulingMailbox,RoomMailbox,EquipmentMailbox” specified by the ValidateSet attribute

Code for example;

[ValidateSet("UserMailbox","SharedMailbox","SchedulingMailbox","RoomMailbox","EquipmentMailbox")]
[String[]]$IncludeMailboxTypes = "UserMailbox"