diff --git a/boxtree/rotation_classes.py b/boxtree/rotation_classes.py
index 97df572cc3994bf7bf1221c087563631337a53fe..4fee3279f0b9ee65918ec13e84247fc7fce64b27 100644
--- a/boxtree/rotation_classes.py
+++ b/boxtree/rotation_classes.py
@@ -132,7 +132,7 @@ TRANSLATION_CLASS_FINDER_TEMPLATE = ElementwiseTemplate(
     // Ensure levels are the same.
     if (box_levels[source_box_id] != box_levels[target_box_id])
     {
-        *error_flag = 1;
+        atomic_or(error_flag, 1);
         PYOPENCL_ELWISE_CONTINUE;
     }
 
@@ -148,7 +148,7 @@ TRANSLATION_CLASS_FINDER_TEMPLATE = ElementwiseTemplate(
     // Ensure valid translation class.
     if (translation_class == -1)
     {
-        *error_flag = 1;
+        atomic_or(error_flag, 1);
         PYOPENCL_ELWISE_CONTINUE;
     }
 
diff --git a/boxtree/tree_build_kernels.py b/boxtree/tree_build_kernels.py
index 3bab74bb6b115fe494294e8b8441d1af779f6d41..9742c4c3f1e5bd3ae38f269c1830e71c1e23021c 100644
--- a/boxtree/tree_build_kernels.py
+++ b/boxtree/tree_build_kernels.py
@@ -878,7 +878,7 @@ LEVEL_RESTRICT_TPL = Template(
                         box_force_split[child_box_id]))
                     {
                         box_force_split[box_id] = 1;
-                        *have_upper_level_split_box = 1;
+                        atomic_or(have_upper_level_split_box, 1);
                         continue_walk = false;
                     }
                 }