ValidationException: The provided key element does not match the schema

I created a table 'user_info' in dynamoDB with one primary hash key 'user_id'(String), no range key. Then I created 2 aws lambda function to insert and query the items. I can insert items into the table, but when I query the table, it returns a ValidationException: The provided key element does not match the schema. My query function :

var params = {
   Key: {
       user_id:{
           S: "usr1@s.com"
       }
   },
    TableName: 'user_info',
    ProjectionExpression: 'password'
};

dynamodb.getItem(params,
 function(err, data) {
    if (err) {
        console.log("get item err." + err);
        context.done('error','getting item from dynamodb failed: '+err);
    }
    else {
        console.log('great success: '+JSON.stringify(data, null, '  '));
        context.succeed('created user ' + event.user_id + ' successfully.');
    }
});

I keep getting this exception:

ValidationException: The provided key element does not match the schema

Since 1)I have only one hash primary key. 2)user_id is defined as String. I really don't know why there is a mismatch error.