How to read a Audio file as binary string in React?

I want to read a file input as a binary string. Therefore I have written a binary string reader (following this post), but I get the error message:

Unhandled Rejection (TypeError): Failed to execute 'readAsBinaryString' on 'FileReader': parameter 1 is not of type 'Blob'.

Reader:

ReadAudioasBinaryString(e) {
    const audio = e.target.files[0];

    return new Promise((resolve, reject) => {
        const reader = new FileReader();

        reader.onload = (event) => {
            resolve(event.target.result);
        };

        reader.onerror = (err) => {
            reject(err);
        };

        reader.readAsBinaryString(audio);
    }, console.log (audio));
} 

Input

<div className="form-group">
         <label>Audio</label>
            <input
              className="form-control"
              type="file"
              name="audio"
              onChange={this.ReadAudioasBinaryString}
              ref={this.inputRef} //
           />
</div>

Happy for every hint!