diff --git a/tools/gn2bp/gn_utils.py b/tools/gn2bp/gn_utils.py
index 5f187a3..4f9060f 100644
--- a/tools/gn2bp/gn_utils.py
+++ b/tools/gn2bp/gn_utils.py
@@ -144,9 +144,9 @@
       # These are valid only for type == 'action'
       self.inputs = set()
       self.outputs = set()
-      self.script = None
+      self.script = ''
       self.args = []
-      self.response_file_contents = None
+      self.response_file_contents = ''
 
       # These variables are propagated up when encountering a dependency
       # on a source_set target.