Amazon MWS SubmitFeed错误:键可能不包含<

我在向Amazon MWS提交提要时遇到问题。我一直收到以下错误:" Invalid query string provided - Keys may not contain < "

这是我的代码:

$apicall = $this->build_url($function, $params);
$ch = curl_init($apicall);
curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);
curl_setopt($ch, CURLINFO_HEADER_OUT, true); 
if ($this->xml) {
    curl_setopt($ch, CURLOPT_POST, true);
    curl_setopt($ch, CURLOPT_POSTFIELDS, (string)$this->xml);
    curl_setopt($ch, CURLOPT_HTTPHEADER, array("Content-type: text/xml"));
    curl_setopt($ch, CURLOPT_HTTPHEADER, array("Content-MD5: " . base64_encode(md5($this->xml))));
}
else {
    curl_setopt($ch, CURLOPT_HTTPHEADER, array('Content-type: text/xml'));
}
curl_setopt($ch, CURLOPT_SSL_VERIFYPEER, true);
$response = curl_exec($ch);
$info = print_r(curl_getinfo($ch), true);
curl_close($ch);

$apicall是动态生成的，其形式为:

https://mws.amazonservices.com/Feeds/2009-01-01?ASINList.ASIN.1=B00C5XBAOA&AWSAccessKeyId=***&Action=SubmitFeed&FeedType=_POST_PRODUCT_PRICING_DATA_&MWSAuthToken=***&SellerId=***&SignatureMethod=HmacSHA256&SignatureVersion=2&Timestamp=2015-06-09T09%3A58%3A01.000Z&Version=2009-01-01&Signature=***

(与其他调用报告或订单的工作良好)

$this->xml在MySQL数据库中保持为"TEXT"字段;这是一个示例XML(我添加了一些行以使其可读):

<?xml version="1.0"?>
<AmazonEnvelope xsi="http://www.w3.org/2001/XMLSchema-instance" noNamespaceSchemaLocation="amzn-envelope.xsd">
    <Header>
        <DocumentVersion>1.01</DocumentVersion>
        <MerchantIdentifier>***</MerchantIdentifier>
    </Header>
    <MessageType>Price</MessageType>
    <Message>
        <MessageID>1</MessageID>
        <OperationType>Update</OperationType>
        <Price>
            <SKU>***</SKU>
            <StandardPrice currency="USD">33.5</StandardPrice>
        </Price>
    </Message>
</AmazonEnvelope>

我似乎在网上浏览了每一个相关的链接，却找不到答案。

也许有人可以给我一个提示，上面的代码可能会出错?

谢谢。