1867, Norman Pinney and Emile Arnoult, Pinney and Arnoult's French Grammar, page 438:
Attendu, ci-inclus, ci-joint, excepté, passé, supposé, compris, franc de port, when they are before a noun, are used as prepositions or adverbial expressions, and are invariable; after the noun they are used as adjectives, and are variable
In this expression the adjective franc is invariable before a noun; but it agrees when the noun precedes.
1948, Maria de Lourdes sá Pereira, Brazilian Portuguese: Grammar, page 59
These numerals are invariable with the exception of um and dois, which have the feminine forms given above.
2006, Rosalind Fergusson and Martin H. Manser, The Complete Guide to Grammar[1]
Singular invariable nouns include mud and impatience, and plural invariable nouns include scissors and trousers. You cannot have *muds, *impatiences, *a scissor, or *a trouser.
