Modifying the default Charge Account in Purchasing

I have setup Purchasing and am using Expense Account Rules (Setup-->Financials-->Accounting-->Expense Account Rules).
When creating requisitions the charge account is the default vendor account defined for the requester at the employee level in HR.
In this combination the account segment is replaced by the value provided in expense account rules.

For e.g

For employee X (Requester) the account combination defined in his employee record is 01.40.40.9999.60.000.000 (9999 is the natural account)
This account is replaced by account in expense account rules i.e say 1132 (an asset account)

Due to cross validation rules defined for an asset account the cost center should be 00 and not 60.