Python3.x Django ValueError：无法将字符串转换为浮点数：''

学习Django与教程和有错误.

File "F:\Python\COTSW\teashop\main\views.py", line 98, in cart_list total_amt+=int(item['qty'])*float(item['price'])个

ValueError: could not convert string to float: ''个

Views.py

def cart_list(request):
    total_amt = 0
    for p_id, item in request.session['cartdata'].items():
        total_amt+=int(item['qty'])*float(item['price'])
    return render(request, 'cart.html', {'cart_data': request.session['cartdata'], 'totalitems': len(request.session['cartdata']), 'total_amt': total_amt})

Product.html

<div class="col-md-8">
            <p>{{data.detail}}</p>
            <hr/>
            <table class="table table-bordered">
                <tr>
                    <h6>Цена</h6>
                    <h6>&#x20BD <span class="product-price-{{data.id}}">{{data.price}}</span></h6>
                </tr>
            </table>
            <hr/>
            <div class="input-group my-3" style="width:30%;">
              <input type="number" value="1" class="form-control product-qty-{{data.id}}" id="productQty" />
              <div class="input-group-append">
                  <input type="hidden" class="product-id-{{data.id}}" value="{{data.id}}" />
                  <input type="hidden" class="product-title-{{data.id}}" value="{{data.title}}" />
                  <input type="hidden" class="product-image-{{data.id}}" value="{{data.image}}" />
                  <input type="hidden" class="product-price-{{data.id}}" value="{{data.price}}" />
                <button class="btn btn-primary btn-sm add-to-cart" data-index="{{data.id}}" type="button" id="addToCartBtn"><i class="fa fa-shopping-cart"></i> В корзину</button>
              </div>
            </div>
        </div>

Custom.js

$(document).on('click', ".add-to-cart", function(){
        var _vm=$(this);
        var _index=_vm.attr('data-index');
        var _qty = $(".product-qty-"+_index).val();
        var _productId = $(".product-id-"+_index).val();
        var _productTitle = $(".product-title-"+_index).val();
        var _productImage = $(".product-image-"+_index).val();
        var _productPrice = $(".product-price-"+_index).text();
        var _productSlug = $(".product-slug-"+_index).val();
        console.log(_productPrice)
        // Ajax
        $.ajax({
            url:'/add-to-cart',
            data:{
                'qty':_qty,
                'id':_productId,
                'title':_productTitle,
                'image':_productImage,
                'price':_productPrice,
                'slug':_productSlug,
            },
            dataType:'json',
            beforeSend:function(){
                _vm.attr('disabled', true);
            },
            success:function(res){
                $(".cart-list").text(res.totalitems);
                _vm.attr('disabled',false);
            }
        });

在更改之前，var _productPrice = $(".product-price").text();是var _productPrice = $(".product-price").text();，并且工作正常.日志(log)显示正确的价格为500.00，但采用空字符串.

教程中的所有内容都运行正常，我不明白哪里会出错.