Thymeleaf get value from radio box does not work

I am trying to get value from radio box to sort my results as a parameter. I targeted "value", however it results in an empty attribute, i.e.:

".../search?searchQuery=hello&charSort="

This is my code, can anyone see wham am I doing wrong?

<div class="panel-body no-gutter-horizontal">
   <div class="checkbox"><label><input type="checkbox" value="size" />Sort by size</label></div>
   <div class="checkbox"><label><input type="checkbox" value="donations" />Sort by donations</label></div>
   <div class="checkbox"><label><input type="checkbox" value="ascending" />Sort by name</label></div>
   <a th:href="@{'/search'(searchQuery=${searchQuery},charSort=${value})}" class="btn btn-large btn-primary">Sort</a>
</div>