Kotlin:类型推断失败。类型参数T的值应在输入类型中提及

我是Kotlin的新手,因为下面的代码:

fun a(stcd: String) {
        val res = mutableSetOf<String>()
        val aaa = mutableListOf<Map<String, Set<String>>>()
        aaa.stream().filter { x: Map<String, Set<String>> -> x.isNotEmpty() }
            .filter { x: Map<String, Set<String>> ->
                x.values.contains(stcd) // throws error
            }.forEach { x: Map<String, Set<String>> ->
                x.forEach { (k: String, v: Set<String>?) ->
                    res.add(k)
                }
            }
    }

有人能指出原因吗 contains 抛出错误: Type inference failed. The value of the type parameter T should be mentioned in input types (argument types, receiver type or expected type). Try to specify it explicitly. ?