diff --git a/var/spack/repos/builtin/packages/serialbox/package.py b/var/spack/repos/builtin/packages/serialbox/package.py
index 625b96567f..8ec0fe3b4c 100644
--- a/var/spack/repos/builtin/packages/serialbox/package.py
+++ b/var/spack/repos/builtin/packages/serialbox/package.py
@@ -156,7 +156,7 @@ def setup_run_environment(self, env):
         env.prepend_path("PATH", self.prefix.python.pp_ser)
         # Allow for running the preprocessor as a Python module, as well as
         # enable the Python interface in a non-standard directory:
-        env.prepend_path("PYTHONPATH", self.prefix.python)
+        env.prepend_path("PYTHONPATH", self.prefix.python.pp_ser)
 
     def setup_dependent_package(self, module, dependent_spec):
         # Simplify the location of the preprocessor by dependent packages: