Table refusing numerical key

I want to create some tables like that :

	liste_planet = {rock_planet = {taille={"minuscule", "petite", "medium", "grande", "énorme"}}}

Work well, apparently.
But if I replace “rock_planet” by a number, here 2, I have an error message :

	liste_rock_planet = {2 = {taille={"minuscule", "petite", "medium", "grande", "énorme"}}}

Message error say :

Compilation failed: '}' expected near '='

But I found examples where a number IS used as key.
What gives ?

Edit : Spelling

Oh, I found an answer :

Said I have to put the number between brackets. Will test that.

Edit : It works.