Labour's shadow education secretary, Angela Rayner, was caught off guard when she was grilled over her party's new policy on class sizes on Wednesday morning (10 May).
Rayner, in a Diane Abbott-style episode with LBC's chief-interrogator Nick Ferrari, failed to say how many pupils would be affected by the plan to slash clash sizes to under 30 for all five, six and seven-year-olds across England.
The exchange (transcribed below) came just hours before Labour leader Jeremy Corbyn unveiled the policy in Leeds.
Labour plan to pay for the initiative by reversing the Conservative Party's cuts to corporation tax, which is due to drop from 20% to 17% by 2020. The move will raise £20bn, the party said.
"It's extremely worrying that the person Jeremy Corbyn wants to put in charge of our children's futures clearly hasn't done her homework," said Conservative schools minister Nick Gibb.
"Corbyn and his top team repeatedly show they cannot cope with basic facts, so just imagine what a mess they would make of negotiating Brexit and running the country.
"Only Theresa May and her Conservative team can provide the strong and stable leadership to provide an excellent education for all our children."
Rayner: "There's quite a substantial amount of pupils that are affected. I haven't got the numbers on me to hand, but it is quite a substantial amount of children that are in class sizes that are over 30."
Ferrari: "Five children or five million children?
Rayner: 'It's a significant number."
Ferrari: "Do you not think it would be a good idea to have a sense of how many people you're talking about?
"I'm asking you how many this will affect and all you can tell me is it is a substantial number?"
"Do you not think you ought to have had that number [520,445]?"
Rayner: "I'm not going to play that numbers game."