Passing an object into a Thymeleaf fragment

I'm attempting to implement a Thymeleaf fragment containing a form, and having trouble passing in the th:object value.

The fragment:

    <div th:fragment="editCard(colSize, title, formObject, formAction, fields)"  th:class="${'col-lg-' + colSize}">
    <div class="card">
        <div class="card-header d-flex align-items-center">
            <h4 th:text="${title}"></h4>
        </div>
        <div class="card-body">
            <form th:object="${__${formObject}__}" th:action="${formAction}" method="post" class="form-horizontal">
                <div th:replace="${fields}"></div>
            </form>
        </div>
    </div>
</div>

And the invocation:

                <div th:replace="fragments/cards2 :: editCard(colSize='12', title='Concert', formObject=${concert}, formAction='/concert/save', fields=~{ :: .fields})" >
                <div class="fields">
                    <input type="hidden" th:field="*{id}"/>
                    <!--/*/ <th:block th:include="fragments/elements :: editSelect(label='Season', val='x', field='season', selectList='${seasons}', itemText='${item}', autofocus='autofocus')"></th:block> /*/-->
                    <!--/*/ <th:block th:include="fragments/elements :: editText(label='Title', val='x', field='title', autofocus='')"></th:block> /*/-->
                    <!--/*/ <th:block th:include="fragments/elements :: editCheckbox(label='Tour', field='tour')"></th:block> /*/-->

                </div>
            </div>
        </div>

There seems to be an interaction between Thymeleaf and Spring binding that prevents the formObject from being accurately passed. I'm not sure exactly which Thymeleaf syntax combination between caller and fragment is correct.