Randall Sell ran in serious problem where query wasn't returning the correct result.
André Knappstein answers:
I can't explain why this happens, but found that - when I experienced the same problems - they all went away when I made a habit of initializing each and every local variable before using it.
Try to explicitely initialize your variable "rate" before the first "select ... into ..." and see if this helps. I definitely did for me (FB 1.5.2, no experience with higher versions because I just initialize all variables always now)
Randall Sell answers:
Well I tried your advise. And to my surprise, when I tested it - it worked! But don't get too excited, after hours more of investigation on the topic, as it turns out, it was a problem with the server. Once I restarted FB, all came good again and order was restored to the universe.
But thanx the advise on good practice. I will keep it in mind.