Someone raised this exactly issue 2 years ago - I found it in the archives.
"Hello,
Using Financial Reporting Studio 11.1.2.4.319. I am having issues with using a substitution variable as a default member when defining prompts in a report. I can select the sub var but when I do, the "$" is removed. When I select OK after defining the prompts I get an "Members in prompt are not valid" error. Sometimes, if I add the "$" manually, it works. Sometimes it does not. Anyone else experience this and know of a solution?
Thanks in advance."
In my case it is reporting server version 11.1.2.4.905.0622 and reporting version 11.1.2.4 with PBCS.
As the original poster advised sometimes it works, othertimes it does not work, and there seems to be no pattern to this and it is not user error.
The only other thing I would add is it is defaulting in without the $ sign and if you use the function selector to repopulate it it does include the $ but that does not necessarily mean that it will then work...
Flakey pastry!
-- sorry, almost forgot to actually ask the question, is anyone else experiencing this and do you know of a fix or consistently working workaround??