diff --git a/faiss/python/class_wrappers.py b/faiss/python/class_wrappers.py
index 71756f5522..2491aa8914 100644
--- a/faiss/python/class_wrappers.py
+++ b/faiss/python/class_wrappers.py
@@ -1038,7 +1038,7 @@ def replacement_vt_train(self, x):
 
 def handle_AutoTuneCriterion(the_class):
     def replacement_set_groundtruth(self, D, I):
-        if D:
+        if D is not None:
             assert I.shape == D.shape
         self.nq, self.gt_nnn = I.shape
         self.set_groundtruth_c(