Magento 2 : Critical exception using WebAPI (Add To Cart)

Question

To reproduce the issue, here are my steps :

• First I get the customer Token using :

http://.../rest/default/V1/integration/customer/token

• I create a cart

http://.../rest/default/V1/carts/mine

• Add add to cart

http://.../rest/default/V1/carts/mine/items

Everything works fine, I can checkout and complete my order.

Then, after this checkout, I want to continue my shopping and do the same steps.

1. Get customer token : OK
2. Create a cart : OK

3. But when I add to cart, I got the following exception :

   [2018-05-11 14:19:03] main.CRITICAL: Report ID: webapi-5af5a657dfda8; Message: Class float does not exist {"exception":"[object] (Exception(code: -1): Report ID: webapi-5af5a657dfda8; Message: Class float does not exist at C:\...\magento2\lib\internal\Magento\Framework\Webapi\ErrorProcessor.php:205, ReflectionException(code: -1): Class float does not exist at C:\...\magento2\lib\internal\Magento\Framework\Reflection\MethodsMap.php:155)"} []

Any idea what could be the issue?

Thanks