diff --git a/.github/workflows/unit-tests.yaml b/.github/workflows/unit-tests.yaml
index 6eb08d942..4277c4955 100644
--- a/.github/workflows/unit-tests.yaml
+++ b/.github/workflows/unit-tests.yaml
@@ -84,6 +84,7 @@ jobs:
             fail-fast: false
             matrix:
                 php: [ '8.1' ]
+        if: github.event_name == 'pull_request' || github.event_name == 'push'
         steps:
             -   name: Checkout
                 uses: actions/checkout@v3
@@ -101,12 +102,20 @@ jobs:
                     coverage: pcov
                     extensions: ctype, iconv, xml
 
+            -   name: Configure the base ref (pull request)
+                if: github.event_name == 'pull_request'
+                run: echo "INFECTION_BASE_REF=$GITHUB_BASE_REF" >> $GITHUB_ENV
+
+            -   name: Configure the base ref (push/release event)
+                if: github.event_name == 'push'
+                run: echo "INFECTION_BASE_REF=${{ github.event.before }}" >> $GITHUB_ENV
+
             # Do this before using the Makefile just to avoid accidentally resolving the deferred variables using
             # INFECTION_FLAGS.
             -   name: Configure Infection for the CI
                 run: |
-                    git fetch origin $GITHUB_BASE_REF
-                    echo "INFECTION_FLAGS=--git-diff-lines --git-diff-base=origin/$GITHUB_BASE_REF --ignore-msi-with-no-mutations --logger-github" >> "$GITHUB_ENV"
+                    git fetch origin $INFECTION_BASE_REF
+                    echo "INFECTION_FLAGS=--git-diff-lines --git-diff-base=origin/$INFECTION_BASE_REF --ignore-msi-with-no-mutations --logger-github" >> "$GITHUB_ENV"
 
             -   name: Install Composer dependencies
                 uses: ramsey/composer-install@v2