diff --git a/scadnano/scadnano.py b/scadnano/scadnano.py
index df7f926..6cfab7d 100644
--- a/scadnano/scadnano.py
+++ b/scadnano/scadnano.py
@@ -6611,13 +6611,19 @@ def _cadnano_v2_place_crossover(helix_from_dct: Dict[str, Any], helix_to_dct: Di
             helix_from_dct[strand_type][end_from - 1][2:] = [helix_to, start_to]
             helix_to_dct[strand_type][end_to-1][:2] = [helix_from, start_from]
             if helix_from_dct["row"]%2 != helix_to_dct["row"]%2:
-                raise ValueError("Paranemic crossovers are only allowed between helices that have the same parity of row number, here helix num "+str(helix_from_dct['num'])+ " and helix num " + str(helix_to_dct['num']) + " have different parity of row number: respectively "+str(helix_from_dct["row"])+" and "+str( helix_to_dct["row"]))
+                raise ValueError("Paranemic crossovers are only allowed between helices that have the same parity of "
+                                 f"row number, here helix num {helix_from_dct['num']} and helix num "
+                                 f"{helix_to_dct['num']} have different parity of row number: respectively "
+                                 f"{helix_from_dct['row']}  and {helix_to_dct["row"]}")
 
         elif not forward_from and not forward_to:
             helix_from_dct[strand_type][start_from][2:] = [helix_to, end_to - 1]
             helix_to_dct[strand_type][end_to-1][:2] = [helix_from, start_from]
             if helix_from_dct["row"]%2 != helix_to_dct["row"]%2:
-                raise ValueError("Paranemic crossovers are only allowed between helices that have the same parity of row number, here helix num "+str(helix_from_dct['num'])+ " and helix num " + str(helix_to_dct['num']) + " have different parity of row number: respectively "+str(helix_from_dct["row"])+" and "+str( helix_to_dct["row"]))
+                raise ValueError("Paranemic crossovers are only allowed between helices that have the same parity of "
+                                 f"row number, here helix num {helix_from_dct['num']} and helix num "
+                                 f"{helix_to_dct['num']} have different parity of row number: respectively "
+                                 f"{helix_from_dct['row']} and {helix_to_dct['row']}")
 
     @staticmethod
     def _cadnano_v2_color_of_stap(color: Color, domain: Domain) -> List[int]: