[Python] Redshift cannot parse response from python lambda

I am trying to create and run a lambda function from within Redshift.

I have scaled the lambda_handler back to something very simple

def lambda_handler(event, context):
print("Received event:", event)

value = 42
print("Returning value:", value, "type:", type(value))

result = [[value]]
print("JSON result to return:", result)

return result


I have also tried setting result as 42 and [42].

My create external function statement is

CREATE OR REPLACE EXTERNAL FUNCTION test_fn()
RETURNS INT
VOLATILE
LAMBDA 'arn:aws:.........'
IAM_ROLE 'arn:aws:iam::........';


However I keep getting the error:

SQL Error [XX000]: ERROR: Invalid External Function Response
Detail:
-----------------------------------------------
error: Invalid External Function Response
code: 8001
context: Cannot parse External Function response
query: 0
location: exfunc_data.cpp:374
process: padbmaster [pid=1073939252]
-----------------------------------------------


I am using a cluster of 2 ra3.large nodes which is patched to version 191.

I am expecting Redshift to return the value 42 when calling SELECT test_fn();

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