diff --git a/fibertree/graphics/tensor_canvas.py b/fibertree/graphics/tensor_canvas.py
index 5b1dd97..ea11020 100644
--- a/fibertree/graphics/tensor_canvas.py
+++ b/fibertree/graphics/tensor_canvas.py
@@ -298,7 +298,7 @@ def addActivity(self,
         highlights_list = []
 
         for hl in highlights:
-            canonical_highlights = HighlightManager.canonicalizeHighlights([hl],
+            canonical_highlights = HighlightManager.canonicalizeHighlights(hl,
                                                                            worker=worker)
 
             highlights_list.append(canonical_highlights)