Image copyright Getty Images Image caption Theresa May surrounded by supporters during a campaign visit to Bridgend

It was a clear election promise. Theresa May said her party was determined to tackle a situation where more than half of mental health problems start below the age of 14.

"So the Conservatives will ensure," said the prime minister, "every primary and secondary school in England and Wales has staff trained in mental health first aid and a single point of contact with local mental health services.

"Under proposed changes to the curriculum, children will be taught more about mental wellbeing, especially in relation to keeping safe online and cyber bullying."

It sounds an interesting policy worthy of scrutiny. But it falls at the first hurdle due to Theresa May's apparent lack of understanding of devolution.

Health and education are run by the Welsh Government in Wales, so was the prime minister proposing to "take back control" of those fields from Westminster?

The Welsh Conservatives were unable to shed much light on the policy so I contacted Conservative Campaign Headquarters, who issued the prime minister's comments.

'Mistake'

And the answer, as is often the case in politics, is more cock-up than conspiracy.

There was, I was told, a mistake in that sentence. A sentence, it's just possible, the prime minister has never seen, but one might expect a party to ensure its press releases in devolved areas are checked for that sort of inaccuracy.

Mrs May is not the only senior politician to make the error of suggesting devolution starts and stops with Scotland and Northern Ireland. (It's a mistake broadcasters make too).

Jeremy Corbyn told his 2016 party conference that: "We will introduce an arts pupil premium to every primary school in England and Wales and consult on the design and national roll-out to extend this pupil premium to all secondary schools."

The Liberal Democrats pointed out that the education secretary in Wales is a Lib Dem. And Welsh Labour politely corrected Mr Corbyn, who presumably won't make the same mistake again.

But with exactly one month to go before polling day, Theresa May's gaffe is unlikely to be the last in this campaign. And if CCHQ is unsure of where the devolution border lies, they've got my number.