etcher: "Error: No polkit authentication agent found" when trying to flash an image as non-root

• Etcher version: 1.0.0-beta.19
• Operating system and architecture:

$ uname -a
Linux patamushka 4.9.0-997-generic #201612270045 SMP Tue Dec 27 05:47:01 UTC 2016 x86_64 GNU/Linux

$ lsb_release -a
No LSB modules are available.
Distributor ID:	Debian
Description:	Debian GNU/Linux 9.0 (stretch)
Release:	9.0
Codename:	stretch

What I do

• open Etcher via CLI by running etcher-electron (as my normal system user)
• select an image I’ve just downloaded from resin.io
• (the disk autopopulates with my SD card)
• click “Flash”

What I expect to happen

For the image to be written to the card

What actually happens

I get the following popup:

{ Error: No polkit authentication agent found.
    at /usr/share/etcher-electron/resources/app.asar/node_modules/sudo-prompt/index.js:132:23
    at ChildProcess.exithandler (child_process.js:225:5)
    at emitTwo (events.js:106:13)
    at ChildProcess.emit (events.js:191:7)
    at maybeClose (internal/child_process.js:877:16)
    at Socket.<anonymous> (internal/child_process.js:334:11)
    at emitOne (events.js:96:13)
    at Socket.emit (events.js:188:7)
    at Pipe._handle.close [as _onclose] (net.js:493:12)
  cause: 
   Error: No polkit authentication agent found.
       at /usr/share/etcher-electron/resources/app.asar/node_modules/sudo-prompt/index.js:132:23
       at ChildProcess.exithandler (child_process.js:225:5)
       at emitTwo (events.js:106:13)
       at ChildProcess.emit (events.js:191:7)
       at maybeClose (internal/child_process.js:877:16)
       at Socket.<anonymous> (internal/child_process.js:334:11)
       at emitOne (events.js:96:13)
       at Socket.emit (events.js:188:7)
       at Pipe._handle.close [as _onclose] (net.js:493:12),
  isOperational: true }

    at Socket.child.stderr.on (/usr/share/etcher-electron/resources/app.asar/lib/child-writer/index.js:129:17)
    at emitOne (events.js:96:13)
    at Socket.emit (events.js:188:7)
    at readableAddChunk (_stream_readable.js:176:18)
    at Socket.Readable.push (_stream_readable.js:134:10)
    at Pipe.onread (net.js:543:20)

After I click OK, another popup appears:

Attention
Oops, seems something went wrong.
[Retry]

Retrying produces the same result.

At the same time as the “Attention” popup, this appears in my system logs:

Mar 11 10:42:30 patamushka dunst[7910]: g_variant_unref: assertion 'value->ref_count > 0' failed
Mar 11 10:42:30 patamushka dunst[7910]: g_variant_unref: assertion 'value->ref_count > 0' failed
Mar 11 10:42:30 patamushka dunst[7910]: g_variant_unref: assertion 'value->ref_count > 0' failed

And this in my auth.log:

Mar 11 10:42:24 patamushka sudo:       aj : a password is required ; TTY=pts/1 ; PWD=/home/aj ; USER=root ; COMMAND=/usr/bin/env ELECTRON_RUN_AS_NODE=1 IPC_SERVER_ID=etcher-server-9763 IPC_CLIENT_ID=etcher-client-9763 SKIP=1 /usr/share/etcher-electron/etcher /usr/share/etcher-electron/resources/app.asar/lib/child-writer/writer-proxy.js /usr/share/etcher-electron/resources/app.asar /home/aj/Downloads/resin-blinky-1.24.1-2.8.3-eef8cf4afe02.img --drive /dev/mmcblk0 --unmount --check

Workaround

By sheer chance, I noticed that if I run a command with sudo–any command, e.g. sudo ls–I’m then able to flash successfully, even though I’m not running etcher-electron with sudo.

If I run via sudo etcher-electron everything works.