diff --git a/.github/workflows/unit_tests.yml b/.github/workflows/unit_tests.yml
index ecc0ffa5804..5256513d790 100644
--- a/.github/workflows/unit_tests.yml
+++ b/.github/workflows/unit_tests.yml
@@ -76,8 +76,8 @@ jobs:
         id: compute-test-matrix
         env:
           compute_all_targets: ${{ contains(github.event.pull_request.title, '[RunAllTests]') }}
-          # See: https://docs.github.com/en/webhooks-and-events/webhooks/webhook-events-and-payloads#pull_request.
-          base_commit_hash: ${{ github.event.pull_request.base.sha }}
+          # See: https://docs.github.com/en/webhooks-and-events/webhooks/webhook-events-and-payloads#pull_request. Defer to origin/develop outside a PR (such as develop's main CI runs).
+          base_commit_hash: ${{ github.event.pull_request.base.sha || 'origin/develop' }}
         # https://unix.stackexchange.com/a/338124 for reference on creating a JSON-friendly
         # comma-separated list of test targets for the matrix.
         run: |