diff --git a/boxtree/rotation_classes.py b/boxtree/rotation_classes.py
index 4fee3279f0b9ee65918ec13e84247fc7fce64b27..58f1c0a82996489a52a5ddb4eb0e5c3a3c072389 100644
--- a/boxtree/rotation_classes.py
+++ b/boxtree/rotation_classes.py
@@ -153,7 +153,7 @@ TRANSLATION_CLASS_FINDER_TEMPLATE = ElementwiseTemplate(
     }
 
     translation_classes[i] = translation_class;
-    translation_class_is_used[translation_class] = 1;
+    atomic_or(&translation_class_is_used[translation_class], 1);
     """)