diff --git a/tests/unit/test_zk.py b/tests/unit/test_zk.py
index b6a05c2045..f3b664da20 100644
--- a/tests/unit/test_zk.py
+++ b/tests/unit/test_zk.py
@@ -1634,9 +1634,17 @@ class TestZKObject(ZooKeeperBaseTestCase):
                                               '/zuul/pipeline/fake_tenant')
             self.assertEqual(pipeline2.foo, 'bar')
 
-        compressed_size, uncompressed_size = pipeline1.estimateDataSize()
+        compressed_size, uncompressed_size = pipeline2.estimateDataSize()
         self.assertTrue(compressed_size != uncompressed_size != 0)
 
+        # Test that nested ZKObject sizes are summed up correctly
+        p1_compressed, p1_uncompressed = pipeline1.estimateDataSize()
+        p2_compressed, p2_uncompressed = pipeline2.estimateDataSize()
+        pipeline2._set(other=pipeline1)
+        compressed_size, uncompressed_size = pipeline2.estimateDataSize()
+        self.assertEqual(compressed_size, p1_compressed + p2_compressed)
+        self.assertEqual(uncompressed_size, p1_uncompressed + p2_uncompressed)
+
         def get_ltime(obj):
             zstat = self.zk_client.client.exists(obj.getPath())
             return zstat.last_modified_transaction_id
diff --git a/zuul/zk/zkobject.py b/zuul/zk/zkobject.py
index 1f2c12b558..3905e254ce 100644
--- a/zuul/zk/zkobject.py
+++ b/zuul/zk/zkobject.py
@@ -210,7 +210,7 @@ class ZKObject:
                 if obj in seen:
                     return 0, 0
                 seen.add(obj)
-                compress, uncompressed = obj.estimateDataSize(seen)
+                compressed, uncompressed = obj.estimateDataSize(seen)
             elif (isinstance(obj, dict) or
                   isinstance(obj, types.MappingProxyType)):
                 for sub in obj.values():