When a variable is specified, it must escape all double quote in its value, for example:
Var invitation="Say \"Hello!\""
This is in standard work flow enforced by the editor which reports error on unescaped double quotes. Variables may be however passed into the tool form various sources (CLI, results of system and I/O commands, etc) and the parser then fails. It should be able to handle such a situation and escape double quotes in the substituted variable calls.