MarshallRequest - NullReference exception

[KarimBenhallam]

I am currently trying to publish a .NET8 backend on AWS Lambda. When testing my lambda function, I get this exact error:

Checking cloudwatch did not help much, as I got the exact same error.

Is anybody familiar with this error? Thank you in advance.

I tried different root paths, different request paths to try and find the source of the error, but I cannot find it.

[bhoradc]

Hi @KarimBenhallam,

Thank you for providing the details and the stack trace. The error Object reference not set to an instance of an object indicates that one of the objects being passed to the MarshallRequest method in the Amazon.Lambda.AspNetCoreServer.APIGatewayProxyFunction class is null, which is causing the NullReferenceException.

Could you please review the MarshallRequest method and identify which of the following objects is potentially null? Once you've identified the null object, you can ensure that it's properly initialized in your Lambda function code.

If you're still unable to resolve the issue after investigating the objects passed to MarshallRequest, please feel free to convert this Discussion to an Issue (or new Issue). And provide necessary details, such a self-contained, concise snippet of code and reproduction steps, which might help us better reproduce and troubleshoot the problem.

Regards,
Chaitanya

[KarimBenhallam]

Hi @bhoradc

This error only intervenes when I test the lambda function on AWS directly. When I build and test my backend endpoints locally, my code runs smoothly. If there is a way to investigate the objects passed to the MarshallRequest method on the AWS interface, I'll be happy to do so.

I tried purposefully calling a non-existing endpoint url to compare behaviours, and I still get the exact same message as earlier, which means the error occurs before the request even reaches my controller.

I'll be happy to follow your debugging instructions, as I am stuck at this point.

Thank you in advance!

[normj]

Can you set the environment variable LAMBDA_NET_SERIALIZER_DEBUG to true on the deployed Lambda function? That will output the incoming Lambda JSON event to CloudWatch logs and that could give us a clue what is null. If possible and nothing sensitive in the JSON it would be great if you could post the JSON here.

[KarimBenhallam]

Can you set the environment variable LAMBDA_NET_SERIALIZER_DEBUG to true on the deployed Lambda function? That will output the incoming Lambda JSON event to CloudWatch logs and that could give us a clue what is null. If possible and nothing sensitive in the JSON it would be great if you could post the JSON here.

It is already set to true, but I see nothing in the cloudwatch logs as shown in my post.

[ashishdhingra]

@KarimBenhallam Good morning. A NullReferenceException (Object reference not set to an instance of an object) within the MarshallRequest method of the APIGatewayProxyFunction typically indicates that the incoming Lambda event is missing expected properties, most often because the API Gateway is not configured correctly as a proxy.
The most common causes include:

• Missing Proxy Integration: The function expects an API Gateway Proxy Integration request. If "Use Lambda Proxy integration" is unchecked in the Amazon API Gateway console, the event payload will lack the structure (like RequestContext or httpMethod) that MarshallRequest needs to map the request to ASP.NET Core.
• Missing RequestContext: The base classes for APIGatewayProxyFunction and APIGatewayHttpApiV2ProxyFunction assume the incoming request contains a non-null RequestContext. If this property is null—often due to manual testing with an incomplete JSON payload in the Lambda Console—the code crashes during marshalling.
• Incompatible Event Type: The error often occurs if you attempt to use APIGatewayProxyFunction (designed for REST APIs) with a Lambda Function URL or an HTTP API. These event sources have different structures that are incompatible with the REST API-specific marshalling logic.
• Incorrect Test Payload: If you are testing via the AWS Lambda Console, ensure you are using the API Gateway AWS Proxy test template. A generic or custom JSON payload that does not include the standard proxy fields will cause a null reference error.
  Environment Variable Issues: Setting certain debug variables, such as LAMBDA_NET_SERIALIZER_DEBUG, can sometimes trigger internal null reference exceptions during response serialization if the handler returns null unexpectedly.

Please review information above and confirm if the issue is resolved at your end.

Thanks,
Ashish