diff --git a/src/silx/gui/plot/ImageStack.py b/src/silx/gui/plot/ImageStack.py
index 175d6e438d..6ca82955be 100644
--- a/src/silx/gui/plot/ImageStack.py
+++ b/src/silx/gui/plot/ImageStack.py
@@ -242,7 +242,7 @@ def reset(self) -> None:
         """Clear the plot and remove any link to url"""
         self._freeLoadingThreads()
         self._urls = None
-        self._urlIndexes = None
+        self._urlIndexes = {}
         self._urlData = {}
         self._current_url = None
         self._plot.clear()