How to use submitter department in purchase order approval condition

Summary:

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I'm trying to use purchase order submitter's department in purchase order approval condition (BPM task DocumentApproval). Finally I assigned a department to my user and I created the simple conditions in BPM to check if submitter's department contains value:

Rule 1:

IF PurchasingDocumentHeader.submitterDepartmentId is null

THEN auto approve

Rule 2:

IF PurchasingDocumentHeader.submitterDepartmentId isn't null

THEN auto reject

When tested the Rule 1 was applied and purchase order was automatically approved. So PurchasingDocumentHeader.submitterDepartmentId is null apparently.

The same happened when using PurchasingDocumentHeader.buyerDepartmentId.

I also tried GetPurchasingDocumentHeaderResponse.result.submitterDepartmentId and GetPurchasingDocumentHeaderResponse.result.buyerDepartmentId but approval finished with error "The approval history can't be displayed because this task couldn't be assigned to any approver."