Week 6 - not getting expected result when searching for flights to FOO

I was following along with the 'Filtering Messages' video in week 6 and noticed a discrepancy. Towards the end of the video, the instructor launches a PostMan request with FOO as the code (http://localhost:8081/flights?code=FOO). In the video he receives a 200 code and an empty list as the returned data. However, when I perform the same thing, I receive an ugly exception message with a code of 500.

I stepped through debugging. The United, American and Delta subflows were all erroring out, which is expected since none have a flight with the destination of FOO. As a result, the output of the Scatter-Gather is a NullPayload, (the exception messages get filtered out because they aren't a List). The issue is the Transform immediately after. When it tries to perform the 'flatten' operation on the NullPayload, it throws an error. Here's what the console says:

ERROR 2017-06-27 14:07:47,628 [[apdev-flights-ws].HTTP_Listener_Configuration.worker.01] org.mule.exception.CatchMessagingExceptionStrategy:

********************************************************************************

Message : Exception while executing:

flatten payload

^

Type mismatch for 'flatten' operator

found :null

required :array.

Payload : {NullPayload}

Payload Type : org.mule.transport.NullPayload

Element : /getAllAirlineFlightsFlow/processors/1 @ apdev-flights-ws:implementation.xml:141 (Transform Message)

Element XML :

<dw:set-payload>%dw 1.0%output application/java---flatten payload</dw:set-payload>

</dw:transform-message>

--------------------------------------------------------------------------------

Root Exception stack trace:

com.mulesoft.weave.mule.exception.WeaveExecutionException: Exception while executing:

flatten payload

^

Type mismatch for 'flatten' operator

found :null

required :array.

Can anyone provide some input as to why this doesn't happen in the videos, but is happening to me? Am I not supposed to get a NullPayload?

I've attached my implementation.xml file - saved as a txt since this forum doesn't allow upload of xml extensions

(NOTE: The Scatter-Gather I have does not call the American flow because I'm behind a strict firewall and unable to call out to the external REST service)

implementation.txt