Some friends - certains or des

Question

The writing challenge asked "Some friends have recommened a gite".
The answer accepted was "Des amis nous ont recommandé un gîte."An answer that was not accepted was "Certains amis nous ont recommandé un gîte."
Why is the second one wrong?

Accepted Answer

Hi David,
There is a difference in meaning between 'Des amis', meaning 'some friends' which is vaguer than 'Certains amis' which would be 'certain friends' more specific in my opinion 
A fine nuance I know...

Answer

Thank you Cécile.
After checking the entries for "some" in Larousse and other dictionaries I can see you are right and "des" the best answer. But is subtle. I am used to Duolingo accepting all possible translations, even the less likely ones.
  
some 
[sʌm] determiner
1.(before uncountable nouns) [a quantity of]  don't forget to buy some cheese/beer/garlic   n'oublie pas d'acheter du fromage/de la bière/de l'ail  let me give you some advice   laissez-moi vous donner un conseil (before plural nouns) [a number of]   des  we've invited some friends round   nous avons invité des amis à la maison  I met some old friends last night   j'ai rencontré de vieux amis hier soir