Null ref on "block" parameter in Word constructor for some results

Expected result:
API result is parsed correctly.

Actual result:
For some document results, the Word constructor fails due to a null ref on the block parameter.
src-csharp/TextractExtensions.cs line 9.
The same document works fine via the web test UI on AWS.

The calling code is : https://github.com/aws-samples/amazon-textract-response-parser/blob/master/src-csharp/TextractExtensions.cs#L318

Looks to be an issue in the API result relating to Relationships, CHILD, WORD and Id?
This happens on API results from completely different source PDFs (all invoices but from separate suppliers).