The number of conditions on the keys is invalid dynamo db with node js

docClient.update({
    TableName: 'patient',
    Key: {
        "patientId": "TIGERPAT0001"
    },
    UpdateExpression: "set title = :x, name = :y",
    ExpressionAttributeNames: {
        "#name": "name"
    },
    ExpressionAttributeValues: {
        ":x": 'title value abc',
        ":y": 'name value xyz'
    }
}, function (err, data) {
    if (err) {
        json.status = '0';
        json.result = { 'error': 'Unable to Edit Patient : ' + JSON.stringify(err) };
        res.send(json);
    } else {
        json.status = '1';
        json.result = { 'sucess': 'Patient Edited Successfully :' };
        res.send(json);
    }
});

when use above code, i got res :

Unable to Edit Patient Error : `{"message":"The number of conditions on the keys is invalid",
"code":"ValidationException", 
"time":"2017-09-13T07:12:56.608Z",
"requestId":"a01c707c-86b4-41a5-a1c5-92b9ea07c026",
"statusCode":400,"retryable":false,
"retryDelay":6.368631970657979}`

What do I miss / any mistake??