diff --git a/generation/poly_hgraph/mol_graph.py b/generation/poly_hgraph/mol_graph.py
index 39f7afd..d827644 100644
--- a/generation/poly_hgraph/mol_graph.py
+++ b/generation/poly_hgraph/mol_graph.py
@@ -13,7 +13,7 @@ class MolGraph(object):
 
     BOND_LIST = [Chem.rdchem.BondType.SINGLE, Chem.rdchem.BondType.DOUBLE, Chem.rdchem.BondType.TRIPLE, Chem.rdchem.BondType.AROMATIC] 
     MAX_POS = 20
-    FRAGMENTS = None
+    FRAGMENTS = []
 
     @staticmethod
     def load_fragments(fragments):