Trying to implement add to cart functionality with Razor but getting the following error when clicking add to cart

Cart Error: 
<error><![CDATA[MESSAGE:
Input string was not in a correct format.
STACKTRACE:
   at System.Number.StringToNumber(String str, NumberStyles options, NumberBuffer& number, NumberFormatInfo info, Boolean parseDecimal)
   at System.Number.ParseInt32(String s, NumberStyles style, NumberFormatInfo info)
   at System.String.System.IConvertible.ToInt32(IFormatProvider provider)
   at System.Convert.ChangeType(Object value, Type conversionType, IFormatProvider provider)
   at TeaCommerce.Presentation.TeaCommerceBase.RequestModule.invokeMethod(restExtension myExtension, Object[] paras)
INNEREXCEPTION:
]]></error>

Here is the Razor script

@using TeaCommerce.Razor
@using TeaCommerce.Data
@inherits umbraco.MacroEngines.DynamicNodeContext
@{
 var products = @Model.Children.Where("!umbracoNaviHide");
  var currentCurrency = TeaCommerce.GetCurrentCurrency();
    <ul>
     @foreach(var item in products)
     {
          var variants = @item.Children;
         <li>
             <form action="/tcbase/teacommerce/SubmitForm.aspx" method="post" name="testformName" id="testformID">
                    <input name="ReturnUrl" type="hidden" value="" />
                    <input name="AddOrderLine" type="hidden" value="nodeId,quantity" />
                  <fieldset>
                       <a href="@item.Url">@item.Name</a>
                     @item.ProductShortDescription
                      
                       <p>
                          Price: @TeaCommerce.FormatPriceWithSpecificCulture(@item.productPriceGBP, @currentCurrency.CultureName)
                        </p>
                     @if(variants.Count() > 0)
                       {
                          <p>
                              <select name="@item.Id">
                                 @foreach(var variant in variants)
                                  {
                                      <option value="@variant.Id">
                                         @variant.Name
                                      </option>
                                    }
                              </select>
                            </p>
                     }
                      else
                       {
                          <input name="nodeId" type="hidden" value="@item.Id" />
                       }
                      <p>
                          Quantity: <input type="text" name="quantity" value="1" />
                            <input type="submit" value="Add to cart" class="productAddToCart" />
                         <img src="/css/gfx/ajax-loader.gif" title="Loading Cart" alt="Loading Cart" />
                       </p>
                 <fieldset>
               </form>
          </li>
        }
  </ul>
}

Any ideas?