Google Apps Script Bad value expextion

I'm getting a "bad value on line 4"... I don't know why. I'm trying to make a Google sheet that automatically opens to an assigned tab based on gmail address for a large team. Please help!

function onOpen() {
  var email = Session.getActiveUser().getEmail();
  var username = email.slice(0,-9);
  var ss = SpreadsheetApp.openById(username);
SpreadsheetApp.setActiveSpreadsheet(ss);
}