diff --git a/pytential/qbx/__init__.py b/pytential/qbx/__init__.py
index 34ab06319764fa2c9efca2860171e376c2a8ea45..f65c31d2cf948be729f63ed23b97b9958b39dcdf 100644
--- a/pytential/qbx/__init__.py
+++ b/pytential/qbx/__init__.py
@@ -243,6 +243,7 @@ class QBXLayerPotentialSource(LayerPotentialSourceBase):
             fmm_order=_not_provided,
             fmm_level_to_order=_not_provided,
             to_refined_connection=None,
+            expansion_factory=None,
             target_association_tolerance=_not_provided,
             _expansions_in_tree_have_extent=_not_provided,
             _expansion_stick_out_factor=_not_provided,
@@ -306,6 +307,8 @@ class QBXLayerPotentialSource(LayerPotentialSourceBase):
                 target_association_tolerance=target_association_tolerance,
                 to_refined_connection=(
                     to_refined_connection or self._to_refined_connection),
+                expansion_factory=(
+                    expansion_factory or self.expansion_factory),
 
                 debug=(
                     # False is a valid value here