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AWS Lambda Invoke — Function Error Fix

DodaTech Updated 2026-06-24 2 min read

In this tutorial, you'll learn about AWS Lambda Invoke. We cover key concepts, practical examples, and best practices to help you understand and apply this topic effectively.

You invoke a Lambda function and get Unhandled, Runtime.ImportModuleError, or Task timed out — the function code has an error, missing dependencies, or incorrect configuration.

Step-by-Step Fix

1. Check the Lambda handler signature

# Wrong: incorrect handler signature (missing context parameter)
def lambda_handler(event):
    print("Got event:", event)
    return {"statusCode": 200}

# Right: correct handler signature with event and context
def lambda_handler(event, context):
    print("Got event:", event)
    print("Request ID:", context.aws_request_id)
    return {"statusCode": 200, "body": "OK"}

2. Test with a simple invoke

# Wrong: verbose error output
aws lambda invoke --function-name my-function --payload '{"key":"value"}' output.txt

# Right: get full error details with log type Tail
aws lambda invoke --function-name my-function --payload '{"key":"value"}' --log-type Tail output.txt

Expected output:

{
    "StatusCode": 200,
    "FunctionError": "Unhandled",
    "LogResult": "base64-encoded-log=="
}

Decode the logs:

aws lambda invoke --function-name my-function --payload '{}' --log-type Tail response.txt | jq -r '.LogResult' | base64 -d

3. Handle exceptions properly

import json
import traceback

def lambda_handler(event, context):
    try:
        name = event['name']
        return {
            'statusCode': 200,
            'body': json.dumps({'message': f'Hello {name}'})
        }
    except KeyError as e:
        print(f"Missing key: {e}")
        return {
            'statusCode': 400,
            'body': json.dumps({'error': f'Missing field: {str(e)}'})
        }
    except Exception as e:
        print(traceback.format_exc())
        return {
            'statusCode': 500,
            'body': json.dumps({'error': 'Internal server error'})
        }

4. Check CloudWatch logs directly

aws logs describe-log-streams --log-group-name /aws/lambda/my-function --max-items 5
aws logs get-log-events --log-group-name /aws/lambda/my-function --log-stream-name "2024/01/15/[$LATEST]abc123"

5. Verify IAM execution role

aws iam get-role --role-name my-function-role-abc123
aws iam list-attached-role-policies --role-name my-function-role-abc123

Expected output should include AWSLambdaBasicExecutionRole at minimum.

Prevention

  • Test Lambda functions locally with SAM CLI or the Lambda Runtime Interface Emulator.
  • Use structured logging with JSON output for easier debugging in CloudWatch.
  • Set environment variables via Lambda console or update-function-configuration.
  • Implement proper error handling with try-catch at the handler level.
  • Use AWS X-Ray for tracing function execution.

Common Mistakes with lambda invoke error

  1. Using return to exit a function early instead of wrapping a pure value in the monad
  2. Mixing let bindings with <- bindings in do notation, producing type errors
  3. Overlapping type class instances that cause GHC to reject the program with ambiguous dispatch errors

These mistakes appear frequently in real-world AWS code. DodaTech's contributors have identified these patterns through analysis of open-source projects and production systems.

Practice Exercise

Write a pure function that safely divides two integers using Maybe, then test it with edge cases like division by zero and negative numbers.

This exercise reinforces the concepts covered in this guide. Try implementing it before checking online solutions.

FAQ

Why do I see "Runtime.ImportModuleError" when invoking?

The Lambda runtime cannot find the module specified in the handler. Ensure the handler path matches the file and export name, and that all dependencies are included in the deployment package. |||How do I see the full error message from a Lambda invocation? Use --log-type Tail with the invoke command, then decode the base64 LogResult. You can also check CloudWatch Logs for the function's log group. |||Why does my function return "Task timed out"? The function exceeded its configured timeout (default 3 seconds). Increase the timeout setting via update-function-configuration --timeout 30 and optimize the code.

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