BadBody: INTERNAL ERROR when input to Backend.Custom.run is mutated

[pyrex41]

when a Backend.Custom function mutates its input, this causes an error from elm-pages: BadBody: INTERNAL ERROR

export async function myFunction(data) {
   for (let key in data) {
      if (data[key] === null) {
        delete data[key];
      }
    };
   ............

the workaround is straight-forward, but would be better I think to handle internally in elm-pages so not an issue to be worried about in custom-backend-tasks.ts

export async function myFunction(dataInit) {
  let data = JSON.parse(JSON.stringify(dataInit));
   for (let key in data) {
      if (data[key] === null) {
        delete data[key];
      }
    };
   ............

[miniBill]

(I suspect this might be the cause for some hangs I've seen in the past)