This error occurs because Blue Prism has specific data types, and Guid isn't one of these, so to get around the issue the code stage needs to be amended so that if a variable is of Guid type, it is first cast to string and then this will be held as a Blue Prism Text type in the collection. Because this involves modifying a VBO provided by Blue Prism, I'd recommend creating a copy and modifying that instead, but when doing so make sure you use the set connection from this ""copy"" object to ensure the global variables are set.
Within the code you will need to use specifier D to maintain the same format that Blue Prism uses (https://msdn.microsoft.com/en-us/library/97af8hh4(v=vs.110).aspx)