diff --git a/src/eynollah/utils/contour.py b/src/eynollah/utils/contour.py
index fb1e790..4b45b03 100644
--- a/src/eynollah/utils/contour.py
+++ b/src/eynollah/utils/contour.py
@@ -353,7 +353,8 @@ def contour2polygon(contour: Union[np.ndarray, Sequence[Sequence[Sequence[Number
     return make_valid(polygon)
 
 def polygon2contour(polygon: Polygon) -> np.ndarray:
-    return np.array(polygon.exterior.coords[:-1], dtype=np.uint)[:, np.newaxis]
+    polygon = np.array(polygon.exterior.coords[:-1], dtype=np.int)
+    return np.maximum(0, polygon).astype(np.uint)[:, np.newaxis]
 
 def make_valid(polygon: Polygon) -> Polygon:
     """Ensures shapely.geometry.Polygon object is valid by repeated rearrangement/simplification/enlargement."""