I was putting together some spreads for a look this morning and got my strikes a little wrong and noticed something strange. The B/A spread for DEC 70/DEC 75 put credit spread is -5.20/-4.80. Isn't 5.00 the maximum risk for this spread at expiration? I'm sure I couldn't get the 5.20 credit, but the 5.00 credit is the mid point. Wouldn't this be a risk free trade? What's the catch? Would I ever get filled? Is the probability of assignment high?