diff --git a/src/py/mat3ra/made/tools/build/slab/builders.py b/src/py/mat3ra/made/tools/build/slab/builders.py
index aad81a45..d7bf5ef6 100644
--- a/src/py/mat3ra/made/tools/build/slab/builders.py
+++ b/src/py/mat3ra/made/tools/build/slab/builders.py
@@ -21,7 +21,7 @@ class SlabBuilder(ConvertGeneratedItemsPymatgenStructureMixin, BaseBuilder):
     _ConfigurationType: type(SlabConfiguration) = SlabConfiguration  # type: ignore
     _GeneratedItemType: PymatgenSlab = PymatgenSlab  # type: ignore
     _SelectorParametersType: type(SlabSelectorParameters) = SlabSelectorParameters  # type: ignore
-    __configuration: SlabConfiguration = None
+    __configuration: SlabConfiguration
 
     def _generate(self, configuration: _ConfigurationType) -> List[_GeneratedItemType]:  # type: ignore
         generator = PymatgenSlabGenerator(