‘Add to basket’ shows as black box on Product page

Hi – the title says it all, really; on the Product page – and only on the Product page, everywhere else is fine – the ‘Add to basket’ box to the right of the Product image shows as a solid black box. Clicking it takes you correctly to the Basket page. The theme is Storefront, & I’m not aware of making any changes that could have affected it; I’ve only just noticed it, but am not sure how long it’s been like it – I’m pretty certain it was OK to start with.

Thanks for any help