Safari could not download the file

A Mac user clicked Export in an app I maintain and got this instead of a spreadsheet: WebKitBlobResource error 1 The pattern that broke The download code was the pattern you find everywhere: fetch the file, turn it into a blob, point an invisible link at it, click the link, tidy up. In outline: async function download(url, name) { const res = await fetch(url); const blob = await res.blob(); const a = document.createElement("a"); a.href = URL.createObjectURL(blob); a.download = name; a.click(); URL.revokeObjectURL(a.href); } The app had four hand-rolled copies of it.