Classed defined in function vs. Top Level one

KotsonTest

    @Test fun json_test_with_nested_data_class() {
        data class MyNestedClass(val name: String, val password: String)

        val x: MyNestedClass = MyNestedClass("hoang", "1234")

        val result = GsonBuilder().create().toJson(x) // -> null
        result.should.equal("""{"name":"hoang","password":"1234"}""")
    }

    data class MyTopLevelClass(val name: String, val password: String)

    @Test fun json_test_with_none_nested_data_class() {
        val x: MyTopLevelClass = MyTopLevelClass("hoang", "1234")

        val result = GsonBuilder().create().toJson(x) // -> ok
        result.should.equal("""{"name":"hoang","password":"1234"}""")
    }