ValueError: View function did not return a valid response

ValueError: View function did not return a valid response

$ flask run
Traceback (most recent call last):
  ...
ValueError: View function did not return a valid response

Why this happens

A view returned None, an unsupported type, or forgot to return anything. Flask expects a string, dict (Flask ≥2.2), tuple (body, status, headers), or a Response object.

Fix

• Ensure every code path returns a valid response.
• Use jsonify for JSON responses, or return tuples when adding status/headers.

Wrong code

from flask import Flask
app = Flask(__name__)

@app.route("/")
def index():
    if False:
        return "OK"
    # Missing return

Fixed code

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

@app.route("/")
def index():
    return jsonify(status="OK")