diff --git a/var/spack/repos/builtin/packages/pbsuite/package.py b/var/spack/repos/builtin/packages/pbsuite/package.py
index ad7a4dd3d22..0b5c22c91f7 100644
--- a/var/spack/repos/builtin/packages/pbsuite/package.py
+++ b/var/spack/repos/builtin/packages/pbsuite/package.py
@@ -25,4 +25,4 @@ def install(self, spec, prefix):
         install_tree('bin', prefix.bin)
 
     def setup_environment(self, spack_env, run_env):
-        run_env.set('PYTHONPATH', self.prefix)
+        run_env.prepend_path('PYTHONPATH', self.prefix)