diff --git a/var/spack/repos/builtin/packages/petsc/package.py b/var/spack/repos/builtin/packages/petsc/package.py
index b2fc80e084..6a7bee9788 100644
--- a/var/spack/repos/builtin/packages/petsc/package.py
+++ b/var/spack/repos/builtin/packages/petsc/package.py
@@ -150,18 +150,19 @@ def install(self, spec, prefix):
 
         # solve Poisson equation in 2D to make sure nothing is broken:
         with working_dir('src/ksp/ksp/examples/tutorials'):
+            cc = os.environ['CC'] if '~mpi' in self.spec else self.spec['mpi'].mpicc  # NOQA: ignore=E501
             os.system('%s ex50.c -I%s -L%s -lpetsc -o ex50' % (
-                self.spec['mpi'].mpicc, prefix.include, prefix.lib))
+                cc, prefix.include, prefix.lib))
             ex50 = Executable('./ex50')
-            ex50('-da_grid_x', '4', '-da_grid_y','4')
+            ex50('-da_grid_x', '4', '-da_grid_y', '4')
             if 'superlu-dist' in spec:
-                ex50('-da_grid_x', '4', '-da_grid_y','4','-pc_type','lu', '-pc_factor_mat_solver_package', 'superlu_dist')  # NOQA: ignore=E501
+                ex50('-da_grid_x', '4', '-da_grid_y', '4', '-pc_type', 'lu', '-pc_factor_mat_solver_package', 'superlu_dist')  # NOQA: ignore=E501
 
             if 'mumps' in spec:
-                ex50('-da_grid_x', '4', '-da_grid_y','4','-pc_type','lu', '-pc_factor_mat_solver_package', 'mumps')  # NOQA: ignore=E501
+                ex50('-da_grid_x', '4', '-da_grid_y', '4', '-pc_type', 'lu', '-pc_factor_mat_solver_package', 'mumps')  # NOQA: ignore=E501
 
             if 'hypre' in spec:
-                ex50('-da_grid_x', '4', '-da_grid_y','4', '-pc_type', 'hypre', '-pc_hypre_type', 'boomeramg')  # NOQA: ignore=E501
+                ex50('-da_grid_x', '4', '-da_grid_y', '4', '-pc_type', 'hypre', '-pc_hypre_type', 'boomeramg')  # NOQA: ignore=E501
 
     def setup_dependent_environment(self, spack_env, run_env, dependent_spec):
         # set up PETSC_DIR for everyone using PETSc package