Another provison for doubtful debts - struggling

I have been through doubtful debts with the AAT e-learning and they provide an attachment to work through.

Debtors 78,780
Have to write off a bad debt of £500
And a provision for doubtful debts needs to be created at 2.5% of debtors balance

So for the adjustments I:

DR bad Debts £500
CR Debtors £500
Bad debt done!

Then I worked out the provision for doubtful debts to be:
(78,780 - 500) x 2.5% = £1,957

The provsion for doubtful debts already had a balance of £6,050. This is where I get a bit stuck. I know I need to use the adjustment figure of 4,093 but I don't understand the answer. I thought the Provision for doubtful debts was credited and the Provision for doubtful debts adjustment account was debited?

So the adjustment would be:
CR Provision for DD £4,093
DR Provision for DD adj £4,093

But the answer is the opposite making the prov. for doubtful debts adj an income in the profit and loss account?

Confused!