Given this Python Flask code snippet for a REST API endpoint:

Given this Python Flask code snippet for a REST API endpoint:

from flask import Flask, request, jsonify
app = Flask(__name__)

@app.route('/submit', methods=['POST'])
def submit():
    data = request.get_json()
    if not data or 'name' not in data:
        return jsonify({'error': 'Missing name field'}), 400
    return jsonify({'message': f"Hello, {data['name']}!"})

What is the likely cause of a 400 Bad Request response?