diff --git a/model_server/base/roiset.py b/model_server/base/roiset.py
index 7cd327ae2403509a5486af4eb145a6514e62dcdd..33c30c1241a06ca7c6da9ba06ad1aa2e98f74c4d 100644
--- a/model_server/base/roiset.py
+++ b/model_server/base/roiset.py
@@ -290,11 +290,16 @@ class RoiSet(object):
 
         # assign labels to object map:
         for i, roi in enumerate(self):
-            oc = np.unique(
-                mask_largest_object(
-                    obmap_patches.iat(i).data
-                )
-            )[-1]
+            try:
+                oc = np.unique(
+                    mask_largest_object(
+                        obmap_patches.iat(i).data
+                    )
+                )[-1]
+            except Exception as e:
+                print(i)
+                print(np.unique(mask_largest_object(obmap_patches.iat(i).data)))
+                raise e
             self._df.loc[roi.Index, 'classify_by_' + name] = oc
             om[self.acc_obj_ids.data == roi.label] = oc
         self.object_class_maps[name] = InMemoryDataAccessor(om)