Errors while upgrading SQL Server 2008 R2 from Standard edition to Enterprise Edition

Task:

Upgrading from 
Microsoft SQL Server 2008 R2 (SP2) - 10.50.4000.0 (X64) 
            Jun 28 2012 08:36:30 
            Copyright (c) Microsoft Corporation
            Standard Edition (64-bit) on Windows NT 6.1 <X64> (Build 7601: Service Pack 1) (Hypervisor)

To 
Microsoft SQL Server 2008 R2 (SP2) - 10.50.4000.0 (X64) nks!
            Jun 28 2012 08:36:30 
            Copyright (c) Microsoft Corporation
            Enterprise Edition (64-bit) on Windows NT 6.1 <X64> (Build 7601: Service Pack 1) (Hypervisor)

Getting the following error:

Rule Check Result

Rule "Upgrade architecture mismatch" failed.

The CPU architectures of upgrading feature(s) and this installation program are different. To upgrade these features, Setup architecture must be the same as the features being installed.

Any help is appreciated.

Thanks

Mahesh