Whoever advised the Prime Minister to go on a late-night American chat show must be regretting it now. David Cameron was twice tripped up on basic knowledge questions - not about the US, but about Britain.
Having been played in to the sound of Rule Britannia, his host, David Letterman, bowled him the first awkward question: who wrote it?
Mr Cameron looked flustered, then plumped for Elgar.
Wrong, it transpired.
The words were by James Thomson and the music by Thomas Arne, as his host pointed out gleefully.
Next came a question on the Magna Carta. The Prime Minister knew what it was and even when it was written, but what did the name actually mean?
It would be good if you knew this, his host suggested. "Yes, it would," said an embarrassed David Cameron.