diff --git a/pycococreatortools/pycococreatortools.py b/pycococreatortools/pycococreatortools.py
index aa6515c..ded5459 100755
--- a/pycococreatortools/pycococreatortools.py
+++ b/pycococreatortools/pycococreatortools.py
@@ -45,8 +45,9 @@ def binary_mask_to_polygon(binary_mask, tolerance=0):
     # pad mask to close contours of shapes which start and end at an edge
     padded_binary_mask = np.pad(binary_mask, pad_width=1, mode='constant', constant_values=0)
     contours = measure.find_contours(padded_binary_mask, 0.5)
-    contours = np.subtract(contours, 1)
+    
     for contour in contours:
+        contour = np.subtract(contour, 1)
         contour = close_contour(contour)
         contour = measure.approximate_polygon(contour, tolerance)
         if len(contour) < 3: