diff --git a/mrdna/segmentmodel.py b/mrdna/segmentmodel.py
index ad49d1220334e18f4596415d68b0bd0116daee5b..3e1978205a1fb73b929f979d9c1644e599e0b80b 100644
--- a/mrdna/segmentmodel.py
+++ b/mrdna/segmentmodel.py
@@ -2514,6 +2514,9 @@ class SegmentModel(ArbdModel):
                 if b1 is b2:
                     return
 
+            if b1 is None or b2 is None:
+                return
+
             ## TODO: improve parameters
             if add_bond:
                 pot = self.get_bond_potential(4,12)