Agreed. Was his only real stumble of the evening and he should have been ready with a snappy one liner that allows him to move on. Aside from that he did pretty well I thought.
May was spluttering variations of her favourite slogans every time she got flustered.
He should still have a better answer ready though. It's obvious he's going to be asked about it and that any equivocation is going to be pounced on.