flow typing activated by if statement doing an "instanceof" check doesn't work as expected with interface types

1 compilation error:

[Static type checking] - Cannot call java.util.Set <CharSequence>#add(java.lang.CharSequence) with arguments [<UnionTypejava.lang.Object+java.lang.CharSequence>] 
 at line: 10, column: 12

@groovy.transform.CompileStatic
class ShowUnionTypeBug {
    Map<String, Object> instanceMap = (Map<String,Object>)['a': 'Hello World']
    def findInstance(String key) {
        Set<CharSequence> allInstances = [] as Set
        def instance = instanceMap.get(key)
        if(instance instanceof CharSequence) {
           allInstances.add(instance)
        }
        allInstances
    }
}
new ShowUnionTypeBug().findInstance('a')