Fetching wikidata properties from an entity discovered by property value

I'm attempting to find location data by the url of an entity. This query returns zero results, but if I replace the P856 clause with ?item wdt:* wd:Q48340 . it works just fine.

SELECT ?item ?itemDescription ?locationLabel ?cityLabel ?postalCodeLabel ?countryLabel ?countryISOCodeLabel ?coordinates ?coordinatesLabel WHERE {
  ?item wdt:P856 "https://www.cnn.com/" .
  OPTIONAL {  }
  OPTIONAL { ?item wdt:P276 ?address. }
  OPTIONAL { ?item wdt:P159 ?headquarters. }
  BIND(COALESCE(?address, ?headquarters) AS ?location)
  OPTIONAL { ?location wdt:P131 ?city. }
  OPTIONAL { ?location wdt:P281 ?postalCode. }
  OPTIONAL { ?location wdt:P17 ?country. }
  OPTIONAL { ?country wdt:P297 ?countryISOCode. }
  OPTIONAL { ?location wdt:P625 ?coordinates. }
  SERVICE wikibase:label { bd:serviceParam wikibase:language "en". }
}

For the entity CNN (Q48340), the wdt:P856 value is an IRI, not a string.

?item wdt:P856 <https://www.cnn.com/> .