No checkout method available

I have OC 1.5.5.1

i operate flat rate shipping with free shipping for over £40 spend..

If a customer buys something over £40 and uses reward points to pay for the item and then selects free shipping during the checkout process they are not allowed to checkout as it states no payment method available.

If they do not use reward points and just checkout as normal there is no problem they just pay via paypal which is the only option i have installed.

Seems like a bug in the code somewhere but i do not know where to look maybe someone on here does?

UPDATE: Just found this workaround:
http://forum.opencart.com/viewtopic.php?t=46375

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I just ran into this issue myself on v1.5.4.1

Does anyone have any ideas on a fix?
It seems this would be a problem for ALL opencart users who use reward points.

If a customer has enough points to get the item for free (with no other purchase), then no payment method is attached, the cart errors out and the user is blocked from completing the transaction.

Makes the merchant look kinda douchy, denying their loyal customers the promised freebie. :/