diff --git a/src/eynollah/utils/contour.py b/src/eynollah/utils/contour.py
index 052688c..393acdd 100644
--- a/src/eynollah/utils/contour.py
+++ b/src/eynollah/utils/contour.py
@@ -20,7 +20,7 @@ def contours_in_same_horizon(cy_main_hor):
     by index into the array.
     """
     sort = np.argsort(cy_main_hor)
-    same = np.diff(cy_main_hor[sort] <= 20)
+    same = np.diff(cy_main_hor[sort]) <= 20
     # groups = np.split(sort, np.arange(len(cy_main_hor) - 1)[~same] + 1)
     same = np.flatnonzero(same)
     return np.stack((sort[:-1][same], sort[1:][same])).T