Annabel Smyth wrote in message

But why does this affect a Travelcard? I don't *have* any PrePay on my
card?

My understanding is that every Oyster card has pre pay, even if
it stays at zero for all its life.

The idea, as I understand it, is that if you have a TC for zones
1,2,3 and use it to go from Z1 to Z4, it will take the Z3-Z4
bit out of your prepay. If this prepay is zero, it will leave
you with a negative prepay balance. This may disable the card for
future use (until you make good the debt) or it may just disable
the out-of-zone aspect; I don't know.

Richard [in PO7]