diff --git a/extensions/th2ex.py b/extensions/th2ex.py
index 2da85da..338ff03 100644
--- a/extensions/th2ex.py
+++ b/extensions/th2ex.py
@@ -815,7 +815,7 @@ def compute_bbox(self, node, transform=True, use_cache=False):
         node_bbox = None
 
         if transform:
-            transform = node.get('transform', '')
+            transform = node.get('transform') or ''
         else:
             transform = ''