diff --git a/segeval/window/windowdiff.py b/segeval/window/windowdiff.py
index 35f8195..21777f3 100644
--- a/segeval/window/windowdiff.py
+++ b/segeval/window/windowdiff.py
@@ -107,7 +107,7 @@ def __window_diff__(hypothesis, reference, window_size, one_minus,
         ref_boundaries = 0
         hyp_boundaries = 0
         # Check that the number of loops is correct
-        assert len(window) is window_size + 1
+        assert len(window) == window_size + 1
         # For pair in window
         for j in range(0, len(window) - 1):
             ref_part, hyp_part = zip(*window[j:j + 2])