Find the Root Cause of a Stack Trace

I have an error I can't immediately explain. Help me find the root cause, not just a
workaround.

Error message: {{error_message}}
Stack trace: {{stack_trace}}
Relevant code (the function(s) in the trace): {{code}}
What I expected to happen: {{expected_behavior}}
What actually happened: {{actual_behavior}}

Do this:
1. Read the stack trace bottom-to-top and identify the exact line where the bad state
   was introduced, not just where it was thrown.
2. List the 2-3 most likely root causes, ranked by likelihood, each with the specific
   evidence from the trace/code that supports it.
3. For the top candidate, give a minimal code change that fixes the cause — not a
   try/catch that hides the symptom.
4. Name one thing I could log or assert to confirm the diagnosis before applying the fix.

If the trace doesn't contain enough information to be confident, say so explicitly and
ask for the specific missing piece (input value, prior log line, version) instead of
guessing.