diff --git a/moses/metrics/utils.py b/moses/metrics/utils.py
index a821b94..b4304bb 100644
--- a/moses/metrics/utils.py
+++ b/moses/metrics/utils.py
@@ -21,7 +21,7 @@
 _pains = pd.read_csv(os.path.join(_base_dir, 'wehi_pains.csv'),
                      names=['smarts', 'names'])
 _filters = [Chem.MolFromSmarts(x) for x in
-            _mcf.append(_pains, sort=True)['smarts'].values]
+            _mcf._append(_pains, sort=True)['smarts'].values]
 
 
 def canonic_smiles(smiles_or_mol):