diff --git a/artiq/test/lit/lit.cfg b/artiq/test/lit/lit.cfg
index b52a4c752..08d1f44b5 100644
--- a/artiq/test/lit/lit.cfg
+++ b/artiq/test/lit/lit.cfg
@@ -19,7 +19,8 @@ else:
     python = sys.executable
 config.substitutions.append( ("%python", python) )
 
-config.environment["PYTHONPATH"] = os.getenv("PYTHONPATH")
+if os.getenv("PYTHONPATH"):
+    config.environment["PYTHONPATH"] = os.getenv("PYTHONPATH")
 
 not_ = "{} {}".format(sys.executable, os.path.join(root, "not.py"))
 config.substitutions.append( ("%not", not_) )