diff --git a/direnv.py b/direnv.py
index 8807eec..51f2b9a 100644
--- a/direnv.py
+++ b/direnv.py
@@ -67,7 +67,7 @@ def _update_environment(self, file_path):
 
         env = {}
         env.update(os.environ)
-        env.update(environment)
+        env.update({k: v for k, v in environment.items() if v is not None})
         returncode, stdout, stderr = get_output(
             ['direnv', 'export', 'json'],
             direnv_path,