diff --git a/src/TorchSisso/FeatureSpaceConstruction.py b/src/TorchSisso/FeatureSpaceConstruction.py
index 14e2d88..2d25311 100644
--- a/src/TorchSisso/FeatureSpaceConstruction.py
+++ b/src/TorchSisso/FeatureSpaceConstruction.py
@@ -13,6 +13,8 @@
 import time
 from itertools import combinations
 from . import combinations_construction
+import re
+from .combinations_construction import FeatureConstructor
 
 class feature_space_construction: