ValidationException：ExpressionAttributeValues不能为空

即使ExpressionAttributeValues不是空的它给我这个错误ValidationException：ExpressionAttributeValues一定不能是空的

app.post('/gpsfromuser', passport.authenticate('jwt', {session: false}), (req, res) => { var xcorpassed = req.body.xcor var ycorpassed = req.body.ycor console.log(xcorpassed); console.log(ycorpassed); var params = { TableName:passengers, Key:{ "pid": req.user.id }, UpdateExpression: "set cordx=:x, cordy=:y", ExpressionAttributeValues:{ ":x":xcorpassed, ":y":ycorpassed }, ReturnValues:"UPDATED_NEW" }; console.log("Updating the item..."); docClient.update(params, function(err, data) { if (err) { console.error("Unable to update item. Error JSON:", JSON.stringify(err, null, 2)); } else { console.log("UpdateItem succeeded:", JSON.stringify(data, null, 2)); return res.status(200).json({msg: "success1"}); } });