diff --git a/.github/workflows/pricing-mutate.yml b/.github/workflows/pricing-mutate.yml
index 953a70c2..4b1afae1 100644
--- a/.github/workflows/pricing-mutate.yml
+++ b/.github/workflows/pricing-mutate.yml
@@ -32,4 +32,4 @@ jobs:
       - name: Run incremental mutation testing
         run: make -C domains/pricing mutate-changes
         env:
-          GIT_BASE_SHA: ${{ github.event_name == 'push' && github.event.before || '' }}
+          GIT_BASE_SHA: ${{ github.event_name == 'push' && github.event.before || github.event.pull_request.base.sha }}
diff --git a/.github/workflows/rails_application-mutate.yml b/.github/workflows/rails_application-mutate.yml
index aef0679f..6f91679b 100644
--- a/.github/workflows/rails_application-mutate.yml
+++ b/.github/workflows/rails_application-mutate.yml
@@ -56,4 +56,4 @@ jobs:
           DATABASE_URL: "postgres://postgres:secret@localhost:5432/cqrs-es-sample-with-res_test"
           RAILS_ENV: test
           MUTANT_OPTS: --jobs 4
-          GIT_BASE_SHA: ${{ github.event_name == 'push' && github.event.before || '' }}
+          GIT_BASE_SHA: ${{ github.event_name == 'push' && github.event.before || github.event.pull_request.base.sha }}
diff --git a/apps/rails_application/Makefile b/apps/rails_application/Makefile
index 20685538..61591a74 100644
--- a/apps/rails_application/Makefile
+++ b/apps/rails_application/Makefile
@@ -26,7 +26,9 @@ else
 	@bundle exec mutant run $(MUTANT_OPTS)
 endif
 
-GIT_BASE_SHA ?= $(shell git merge-base HEAD origin/master)
+# Fall back to merge-base when GIT_BASE_SHA is empty or the git null SHA
+# (push of a new branch sets github.event.before to 0000…0).
+GIT_BASE_SHA := $(or $(filter-out 0000000000000000000000000000000000000000,$(GIT_BASE_SHA)),$(shell git merge-base HEAD origin/master))
 
 mutate-changes: ## Run incremental mutation tests on changes
 	@echo "Running incremental mutation tests against $(GIT_BASE_SHA)..."
diff --git a/domains/pricing/Makefile b/domains/pricing/Makefile
index 7382e574..91de9de2 100644
--- a/domains/pricing/Makefile
+++ b/domains/pricing/Makefile
@@ -1,4 +1,6 @@
-GIT_BASE_SHA ?= $(shell git merge-base HEAD origin/master)
+# Fall back to merge-base when GIT_BASE_SHA is empty or the git null SHA
+# (push of a new branch sets github.event.before to 0000…0).
+GIT_BASE_SHA := $(or $(filter-out 0000000000000000000000000000000000000000,$(GIT_BASE_SHA)),$(shell git merge-base HEAD origin/master))
 
 install:
 	@bundle install