收集字符集流

我有这个字符集流:

Stream<Set<Character>> characterSet

现在,我想将此流转换为简单的字符集,然后执行以下操作:

Set<Character> newSet = characterSet.collect(Collectors.toSet());

但是,我收到此错误:

Incompatible types. Required Set<Character> but 'collect' was inferred to R: no instance(s) of type variable(s) exist so that Set<Character> conforms to Character inference variable T has incompatible bounds: equality constraints: Character lower bounds: Set<Character>.

我该如何解决?谢谢!