According to the NACHA spec, Addenda Sequence Number (position 84-87) should always start with '1'

jgoldberg · jgoldberg · commit 9029d3a3f154 · 2015-09-15T09:51:55.000-05:00
diff --git a/ach/builder.py b/ach/builder.py
@@ -327,7 +327,7 @@ def __init__(self, entry_detail, addenda_record=[]):
                 AddendaRecord(
                     self.entry_detail.std_ent_cls_code,
                     pmt_rel_info=addenda.get('payment_related_info').upper(),
-                    add_seq_num=index,
+                    add_seq_num=index + 1,
                     ent_det_seq_num=entry_detail.trace_num[-7:]
                 )
             )

Original file line number	Diff line number	Diff line change
`@@ -327,7 +327,7 @@ def __init__(self, entry_detail, addenda_record=[]):`
`327`	`327`	`AddendaRecord(`
`328`	`328`	`self.entry_detail.std_ent_cls_code,`
`329`	`329`	`pmt_rel_info=addenda.get('payment_related_info').upper(),`
`330`		`- add_seq_num=index,`
	`330`	`+ add_seq_num=index + 1,`
`331`	`331`	`ent_det_seq_num=entry_detail.trace_num[-7:]`
`332`	`332`	`)`
`333`	`333`	`)`