"fatal: remote error: User 'None' does not have write permissions for repository" on initial push

I'm trying to create a new sandbox project, but I'm having problems getting code up to the git.drupal.org git server. Whenever I try to perform my first push, I get an error that "user 'None' does not have write permissions for the repository."

I've followed the directions in the Empty Sandbox Repository page, but to no avail:

$ git init
Initialized empty Git repository in /path/to/import_export_tools/.git/
$ echo "name = Import / Export Tools" > import_export_tools.info
$ git add import_export_tools.info
$ git commit -m "Initial commit."
[master (root-commit) 2ae198b] Initial commit.
 1 files changed, 1 insertions(+), 0 deletions(-)
 create mode 100644 import_export_tools.info
$ git remote add origin franklin@git.drupal.org:sandbox/franklin/1255136.git
$ git push --verbose origin master
Pushing to franklin@git.drupal.org:sandbox/franklin/1255136.git
fatal: remote error: User 'None' does not have write permissions for repository '1255136'

I can confirm the SSH keys are correct: Attempts to SSH to git.drupal.org using ssh -v git.drupal.org succeed with the debug message Authentication succeeded (publickey). Appropriately, no shell is granted, but the authentication succeeds.

Near as I can tell, the git config is correct; 'franklin' is my Git username.

$ git config --list
user.name=John Franklin
user.email=franklin@sentaidigital.com
core.repositoryformatversion=0
core.filemode=true
core.bare=false
core.logallrefupdates=true
remote.origin.url=franklin@git.drupal.org:sandbox/franklin/1255136.git
remote.origin.fetch=+refs/heads/*:refs/remotes/origin/*

and my ssh config configures connections correctly:

$ cat ~/.ssh/config
Host git.drupal.org
User franklin
IdentityFile ~/.ssh/drupal

What am I missing? Any ideas?