@AaronSadlerUK but you don't really want

null

you want

"null"

. Right? i.e. you actually want to append the string "null" to the URI and you are using a type conversion to turn

null

into

"null"

. Rather than rely on this conversion/convention it'd be better to be explicit, i.e.

culture = culture || "null";

Or, If you're only targeting modern browsers...

javascript
function (id, culture = "null") {
  return $http.get(`backoffice/UmbNav/UmbNavEntityApi/GetById?id=${id}&culture=${culture}`)
  \\...