Problems properly decoding a base64 encoded string

I am trying to decode an Azure Storage Key that’s supposedly base64 encoded, but I’m getting all kinds of wonkey characters. Example:

Base.decode64! "ZG2p8/z2ATu8NVLBXmeFtkX9chpkbk2EK9fH4qVKxvC//mSlIdsqzvZpNa4k7tPPR86RmiGkdi0BLwAEtPbkXA==" |> String.codepoints

Results in:

[
  "d",
  "m",
  <<169>>,
  <<243>>,
  <<252>>,
  <<246>>,
  <<1>>,
  ";",
  <<188>>,
  "5",
  "R",
  <<193>>,
  "^",
  "g",
  <<133>>,
  <<182>>,
  "E",
  <<253>>,
  "r",
  <<26>>,
  "d",
  "n",
  "M",
  <<132>>,
  "+",
  <<215>>,
  <<199>>,
  <<226>>,
  <<165>>,
  "J",
  <<198>>,
  <<240>>,
  <<191>>,
  <<254>>,
  "d",
  <<165>>,
  "!",
  <<219>>,
  "*",
  <<206>>,
  <<246>>,
  "i",
  "5",
  <<174>>,
  "$",
  <<238>>,
  <<211>>,
  <<207>>,
  "G",
  "Α",
  ...
]

This definitely isn’t correct, or I’m not thinking about something correctly here. Is there something I need to do special for utf8 encoded strings?

Thanks for the help,
Jeramy