I would say because 'cream': 0 should not be included in the recipe. If you omit it, you have a non zero solution... based on most code, cream:0 would cause an exception.

I am not sure why the correct answer is 0 here...
recipe: { cream: 0, oil: 97, pears: 43 }
available: { nuts: 7300, eggs: 2800, butter: 2900, flour: 3000, oil: 4000, cream: 4000, apples: 200, pears: 9700, cocoa: 1900, milk: 4100, sugar: 3400, crumbles: 8300, chocolate: 5800 }
Can someone please explain to me! Thanks!