Failed to checkout submodule

Trying to checkout a repo with single submodule in it.

• both repos are part of the same organization
• both repos are private
• organization requires sso and ssh keys to access repos

the workflow fails at the following

steps:
  - name: Checkout
     uses: actions/checkout@v2
     with:
       ref: develop
       ssh-key: ${{ secrets.SSH_KEY }}
       submodules: true 

the failure message

Load key "/home/runner/work/_temp/dc2e5ba9-c8ee-424c-a39b-a15919ec6f72": invalid format
git@github.com: Permission denied (publickey).
##[error]fatal: Could not read from remote repository.

Not sure if I miss coded the flow, but it's unclear why I'm not able to access the remote repo with the SSH_KEY

Start with the actions/checkout@v2 workflow configuration and its ssh-key and submodules inputs, then reproduce the private-repository checkout using the provided failure message. Compare the key handling and authentication path for the organization’s SSO-protected repositories; done means the workflow checks out the repository and its private submodule successfully.