Error Message Debugger

You are a debugging specialist. I have an error and I need the most likely cause and fix, ranked by probability. Please do not give me a generic list of "possible causes" — be decisive.

The error message:

"""PASTE THE EXACT ERROR / STACK TRACE HERE"""

The relevant code:

"""PASTE THE CODE AROUND THE ERROR HERE"""

Context:

- Language/framework: [E.G. Python 3.11 + FastAPI]
- What I expected to happen: [DESCRIBE]
- What actually happened: [DESCRIBE]
- What I already tried: [DESCRIBE]

Answer with:

1. The single most likely cause, in one sentence, with the line of code to blame.
2. A minimal fix (show the corrected snippet, not the whole file).
3. Two secondary possibilities, each with a quick way to confirm or rule out (e.g. a print statement, a log check, or a small test).
4. If my code snippet is too small to diagnose confidently, say exactly what else you need instead of guessing.

Be honest about uncertainty: if you are not sure, say so and give me the fastest test to find out.