Rick, it is grammatically wrong. 

However, you may sometimes hear it in informal speech, just as grammatical 'errors' are heard everyday in English !

 "Des" becomes " de/d' " in front of adjectives preceding nouns (French Partitive Articles) 

PS: I just left answering this question to start reading an article in a French newspaper (L'Est Républicain)  - saw this example sentence highlighting what happens with contraction of de and les, as is noted also in the attached lesson - hors de 'les grandes villes' - "hors des grandes villes". Thought it might be interesting and reinforcing ! 

En 2019, 55 % des travailleurs francs-comtois vivent hors des grandes villes. 

PPS: Same 'journal', different article, example of the grammar point questioned initially 

" (ils) .. se sont contentés d'exhiber de larges draps peints des lettres de ... "

It's wrong.

'Des' becomes 'de' before an adjective + noun:

"Des" becomes " de/d' " in front of adjectives preceding nouns (French Partitive Articles)