Integrator.io/celigo fulfillment TypeError: Cannot read property 'fulfillable_quantity' of undefined

[ddickerson]

I'm reaching out here as well as the Celigo community for any help.
My company uses integrator.io to connect Shopify and Acumatica, and my issue comes up during about 5 percent of Acumatica shipments to Shopify Fulfillment orders.  

On these orders I get an error stating "TypeError: Cannot read property 'fulfillable_quantity' of undefined" during a Post response map hook. 

I can't find a common link between the errored orders or items on the orders being error. 

I had a Celigo tech look at it and they added a null check, but this also errored and seemed to skip over some inventory items during the fulfillment. 

Has anyone else run in to this error or a similar error?