AWS S3 x-amz-meta error with non-string values: InvalidHeader and InvalidParameterType

When attempting to use the node.js AWS sdk s3Client.upload with optional parameters for metadata:

const AWS = require('aws-sdk')
const s3Client = new AWS.S3({
  params: {
    Key: key,
    secret: secret
  }
})

let uploadParams = {
  Bucket: '',
  Key: '',
  Body: '',
  ACL: 'public-read',
  Metadata: {
    dummy_value: null
  }

s3Client.upload(uploadParams, function(err, data) {
  console.log('Upload: ', data)
}

Error is thrown:

InvalidHeader: Header x-amz-meta-dummy_value contains invalid value