diff --git a/.github/workflows/coverage.yml b/.github/workflows/coverage.yml
index 04211f5..4b83bd5 100644
--- a/.github/workflows/coverage.yml
+++ b/.github/workflows/coverage.yml
@@ -49,7 +49,9 @@ jobs:
       - name: Patch coverage gate (100% of changed lines)
         if: github.event_name == 'pull_request'
         run: |
-          git fetch origin "${{ github.base_ref }}" --depth=1 || true
+          # Full base history (NOT --depth=1): a shallow base fetch shares no merge-base
+          # with a PR branch that is behind base → diff-cover "no merge base" crash.
+          git fetch origin "${{ github.base_ref }}" || true
           diff-cover coverage/lcov.info \
             --compare-branch="origin/${{ github.base_ref }}" \
             --fail-under=100