diff --git a/python/tidy/servo_tidy/tidy.py b/python/tidy/servo_tidy/tidy.py
index 1336c4d0b56..d0133dcd05d 100644
--- a/python/tidy/servo_tidy/tidy.py
+++ b/python/tidy/servo_tidy/tidy.py
@@ -316,7 +316,7 @@ def check_flake8(file_name, contents):
 
 def check_lock(file_name, contents):
     def find_reverse_dependencies(name, content):
-        for package in itertools.chain([content["root"]], content["package"]):
+        for package in itertools.chain([content.get("root", [])], content["package"]):
             for dependency in package.get("dependencies", []):
                 if dependency.startswith("{} ".format(name)):
                     yield package["name"], dependency