diff --git a/src/silx/app/compare/CompareImagesWindow.py b/src/silx/app/compare/CompareImagesWindow.py
index 980abfd7d6..0f31885430 100644
--- a/src/silx/app/compare/CompareImagesWindow.py
+++ b/src/silx/app/compare/CompareImagesWindow.py
@@ -138,7 +138,7 @@ def readData(self, urlPath: str):
 
         if data.ndim == 2:
             return data
-        if data.ndim == 3 and data.shape[0] in set(3, 4):
+        if data.ndim == 3 and data.shape[0] in set([3, 4]):
             return data
 
         raise ValueError(f"URL '{urlPath}' does not link to an numpy image")