diff --git a/openmc/surface.py b/openmc/surface.py
index ca1501b887..5dc25103f3 100644
--- a/openmc/surface.py
+++ b/openmc/surface.py
@@ -25,8 +25,6 @@ _WARNING_KWARGS = """
 will not accept positional parameters for superclass arguments.\
 """
 
-_SURFACE_CLASSES = {}
-
 
 class Surface(IDManagerMixin, metaclass=ABCMeta):
     """An implicit surface with an associated boundary condition.