Node AWS-lib: repeated calls to AWS-lib causes bad signature

I am running the same EC2 API call repeatedly, as I wait for an instance to be started.

var check_started = function() {
  console.log('Calling');
  ec2.call("DescribeInstanceStatus", {InstanceId:['pretend_instance_id']}, function(err, status_result){
    if (err) {
      console.log('error')
      console.log(err)
      console.log(status_result)
    } else {
      console.log('success')
    }
  }); 
}

var instance_started_checker = setInterval( check_started, 5 * 1000)

The first call always succeeds, but subsequent calls fail with:

The request signature we calculated does not match the signature you provided. Check your AWS Secret Access Key and signing method. Consult the service documentation for details.

What can I do to make the repeated calls work?

I think it might help if you specify an API version in the options of creating an agent. Like so:

var ec2 = aws.createEC2Client(yourAccessKeyId, yourSecretAccessKey, {
        version: "2012-04-01"
    }
);

I tried your code, but was not able to reproduce the same error. I get 'InvalidAction' when I do not specify the API version. This is the code I used:

var aws = require("aws-lib");
var ec2 = aws.createEC2Client("xXx", "yYy", {
    secure: "https",
    host: "ec2.eu-west-1.amazonaws.com",
    version: "2012-04-01"
}
);
var check_started = function() {
console.log('Calling');
ec2.call("DescribeInstanceStatus", {InstanceId:["i-abcdefg"]}, function(err, status_result) {
    if (err) {
      console.log('error')
      console.log(err)
      console.log(status_result)
    } else {
      console.log('success')
      console.log(status_result.instanceStatusSet)
      clearInterval(instance_started_checker);
    }
});
}
var instance_started_checker = setInterval( check_started, 5 * 1000);

Out of curiosity, do you get the same error with invoking DescribeInstances?