MissingMethodException on groovy script

[Pankaj Mehta]

The below code i am using to create a link back to the jira's issue.

// write storage format using an XML builder

def writer = new StringWriter()

def xml = new MarkupBuilder(writer)

xml.'ac:structured-macro'('ac:name': "jira") {

    'ac:parameter'('ac:name': "key", issue.key)

    'ac:parameter'('ac:name': "server", "System JIRA")

    'ac:parameter'('ac:name': "serverId", "YOUR-SERVER-ID")

}

and i am getting the below error on script console and also i imported the markup builder class.:

groovy.lang.MissingMethodException: No signature of method: java.lang.String.ac:structured-macro() is applicable for argument types: (LinkedHashMap, Script105$_run_closure1) values: [[ac:name:jira], Script105$_run_closure1@100d93a9] at Script105.run(Script105.groovy:47)

what is the wrong here or which method is missing?