I have a checkbox for my form configured like this:

<div class="form-check">
    <input type="hidden" name="tx_form_formframework[cf-9][checkbox-1]" value="">
    <input required="required" class="add-on form-check-input" id="cf-9-checkbox-1" type="checkbox" name="tx_form_formframework[cf-9][checkbox-1]" value="1">
    <label class="add-on form-check-label" for="cf-9-checkbox-1">
        <span>Text<span class="required">*</span></span>
    </label>
</div>

Everytime when I check or uncheck it, it outputs the following error message:

It still works fine but I can't figure out why this error occours and if it will cause any problems.