diff --git a/kraken/lib/segmentation.py b/kraken/lib/segmentation.py
index 10f82941b..0685dfc3f 100644
--- a/kraken/lib/segmentation.py
+++ b/kraken/lib/segmentation.py
@@ -357,7 +357,7 @@ def vectorize_regions(im: np.ndarray, threshold: float = 0.5):
     if boundaries.type == 'Polygon':
         boundaries = [boundaries.boundary.simplify(10)]
     else:
-        boundaries = [x.boundary.simplify(10) for x in unary_union(boundaries)]
+        boundaries = [x.boundary.simplify(10) for x in boundaries.geoms]
     return [np.array(x.coords, dtype=np.uint)[:, [1, 0]].tolist() for x in boundaries]