diff --git a/tests/regression_tests/cpp_driver/test.py b/tests/regression_tests/cpp_driver/test.py
index 6870cfe7d7..95912b3f7b 100644
--- a/tests/regression_tests/cpp_driver/test.py
+++ b/tests/regression_tests/cpp_driver/test.py
@@ -40,7 +40,7 @@ def cpp_driver(request):
     try:
         print("Building driver")
         # Run cmake/make to build the shared libary
-        subprocess.run(['cmake', os.path.pardir, '-DOPENMC_USE_MPI=' + mpi_arg], check=True)
+        subprocess.run(['cmake', os.path.pardir, f'-DOPENMC_USE_MPI={mpi_arg}'], check=True)
         subprocess.run(['make'], check=True)
         os.chdir(os.path.pardir)
 
diff --git a/tests/regression_tests/external_moab/test.py b/tests/regression_tests/external_moab/test.py
index da55c735a9..47e3ef80ed 100644
--- a/tests/regression_tests/external_moab/test.py
+++ b/tests/regression_tests/external_moab/test.py
@@ -56,7 +56,7 @@ def cpp_driver(request):
     try:
         print("Building driver")
         # Run cmake/make to build the shared libary
-        subprocess.run(['cmake', os.path.pardir, '-DOPENMC_USE_MPI=' + mpi_arg], check=True)
+        subprocess.run(['cmake', os.path.pardir, f'-DOPENMC_USE_MPI={mpi_arg}'], check=True)
         subprocess.run(['make'], check=True)
         os.chdir(os.path.pardir)