Netlify form submits but shows "Error: --success msg--" after form submission

Question

When I submit the form, I get the response in my mail box but I see something like..

Error: Thank You! after I submit it.

Here is my code:

<form action="./submitted.html" name="contact-form" method="POST" role="form" class="php-email-form"
    data-netlify="true">
    <div class="row">
        <div class="form-group col-md-6">
            <label for="name">Your Name</label>
            <input type="text" name="name" class="form-control" id="name" required>
        </div>
        <div class="form-group col-md-6">
            <label for="name">Your Email</label>
            <input type="email" class="form-control" name="email" id="email" required>
        </div>
    </div>
    <div class="form-group">
        <label for="name">Subject</label>
        <input type="text" class="form-control" name="subject" id="subject" required>
    </div>
    <div class="form-group">
        <label for="name">Message</label>
        <textarea class="form-control" name="message" rows="10" required></textarea>
    </div>
    <div class="my-3">
        <div class="loading">Loading</div>
        <div class="error-message"></div>
        <div class="sent-message">Your message has been sent. Thank you!</div>
    </div>
    <div class="text-center"><button type="submit">Send Message</button></div>
</form>

And here is the output I see after form submission. Output after form submission

What I don't know is how am I getting the Error: word before my success message