diff --git a/taskcluster/taskgraph/optimize/__init__.py b/taskcluster/taskgraph/optimize/__init__.py
index 71134807671e..aac5b476cb24 100644
--- a/taskcluster/taskgraph/optimize/__init__.py
+++ b/taskcluster/taskgraph/optimize/__init__.py
@@ -374,7 +374,7 @@ class All(CompositeStrategy):
     @classmethod
     def reduce(cls, results):
         for rv in results:
-            if !rv:
+            if not rv:
                 return rv
         return True