Screenshot: Invalid base64


When I use:
let screenshot_uu = await driver.takeScreenshot();

the returned string is exactly 100_000 characters - suspicious but not invalid - and characters 50_000 - 50_002 are e2 80 a6 which are not in the valid set for base64 as far as I know.

atob throws a exception on the string

thDOMException [InvalidCharacterError]: Invalid character
    at atob (node:buffer:1242:7)

On inspection that is what I found those characters above in the string

Have I done something wrong in the way I call takeScreenshot? Would more context help?