Certificate Satisfaction Excluded Exceptions in Grails

I am having trouble acquiring the CertificateExpiredException in Grails. When I add an exception to the code, I get "The parameter type of the 'Catch statement' is not a Throwable subclass". message. If I use the generic Exception parameter as the catch parameter it works. How can I catch this SSL error and give a meaningful response to the user?

Here's an example of a service:

class MyService {

    static transactional = false

    String someUrl = 'https://example.com'

    def getThings() {

        def conn
        try {
            conn = someUrl.toURL().openConnection()
        } catch (CertificateExpiredException e) { // doesn't like this
            log.debug e // doesn't work?
            return "SSL error, no results returned."
        }

        if(conn.responseCode == 200){
            // do stuff
        }
    }
}