Conflict in replacing permissions in impersonation step of workflow

Hi

I have document library with two additional columns added to it:

1. Add unique ownership permission to these users

2. Add unique contribute permission to these users

I want to change Full control and Contribute permission of document with values specified in these columns when docuemtn is checked in. If these values are kept blank respective permissions will be assiegned to site owners and site members.

I am using following steps in impersonation step of workflow. The workflow runs when item is created:

If Current Item: add unique ownership to these users not equals variable:blank
        Replace Full Control of item in Current Item
        Log added unique full control permission
Else
        Replace Full Control of item in Current Item
        Log added default Full Control permission
If Current Item: add unique contribute to these users not equals variable:blank
        Replace Contribute of item in Current Item
        Log added unique contribute permission
Else
        Replace Contribute of item in Current Item
        Log added default Contribute permission

This workflow is not running as per the expectations. This either throws an error or assign permissions in an undesired way. Please advise how I shall correct it.