diff --git a/waflib/Utils.py b/waflib/Utils.py
index 7472226d..fc64fa05 100644
--- a/waflib/Utils.py
+++ b/waflib/Utils.py
@@ -891,7 +891,7 @@ def run_prefork_process(cmd, kwargs, cargs):
 	"""
 	Delegates process execution to a pre-forked process instance.
 	"""
-	if not 'env' in kwargs:
+	if not kwargs.get('env'):
 		kwargs['env'] = dict(os.environ)
 	try:
 		obj = base64.b64encode(cPickle.dumps([cmd, kwargs, cargs]))