diff --git a/boxtree/tree_build.py b/boxtree/tree_build.py
index 8f652aa8397787b35199e74d18bb2ef041697eb9..38c69b7e38e0011d5652a01c9c7856678b2df70b 100644
--- a/boxtree/tree_build.py
+++ b/boxtree/tree_build.py
@@ -220,7 +220,16 @@ class TreeBuilder(object):
 
         def zeros(shape, dtype):
             result = cl.array.zeros(queue, shape, dtype, allocator=allocator)
-            event, = result.events
+            if result.events:
+                event, = result.events
+            else:
+                from numbers import Number
+                if isinstance(shape, Number):
+                    shape = (shape,)
+                from pytools import product
+                assert product(shape) == 0
+                event = cl.enqueue_marker(queue)
+
             return result, event
 
         knl_info = self.get_kernel_info(dimensions, coord_dtype,