"The Master's TARDIS," she explains, sobering a little at the reminder that she ought to start being careful about spoilers around him. With all the excitement it had actually almost slipped her mind. "We met under... somewhat peculiar circumstances, so I could not tell when she was from." Just so he doesn't get any ideas about the Master and his TARDIS still roaming around. It may not be as risky to give things away to this Doctor, since he's from a different universe where events can easily play out differently, but she still prefers to be cautious.