Replace in string

Hello,
I have a form that is used to email. I have a field called "SendTo", which is a look up to a table with email addresses. In the properties of this look up field I have "Allow multiple selection" checked. When I run the form, if i select multiple email addresses it produces a comma separated list. I'd like to replace the comma with a semicolon and then use this field to populate the "string email =" field used by the Runner email function. Here is what I've tried on the "Before record added" event:
string email = values["SendTo"].Replace(",",";");
This doesn't seem to work. How should I do this?
(Let me know if you need more info).
Thanks,

Tim