TypeError: Failed to execute 'readAsText' on 'FileReader': parameter 1 is not of type 'Blob'. code example

Example: core.js:4352 ERROR TypeError: Failed to execute 'readAsArrayBuffer' on 'FileReader': parameter 1 is not of type 'Blob'.

onSelectFile(event) {
    let reader = new FileReader();
    reader.onload = function(){
      let output: any = document.getElementById('blah');
      output.src = reader.result;
    }
    if(event.target.files[0]){
      reader.readAsDataURL(event.target.files[0]);
    }
}