diff --git a/svreader/annotation.py b/svreader/annotation.py
index 11c2b9855d973ca60e8159a1a7e85a431ae0fc34..6d2363ebc55547ef6cc0741c60cff3c53ff5fec2 100644
--- a/svreader/annotation.py
+++ b/svreader/annotation.py
@@ -616,7 +616,7 @@ def SetSupportingTools(SVSet):
                 tools.add(get_tool_name(dupli))
         if 'TOOLSUPPORT' in sv.record.info:
             supporting = set(sv.record.info['TOOLSUPPORT'])
-            tools.union(supporting)
+            tools = tools.union(supporting)
         sv.record.info['TOOLSUPPORT'] = list(tools)