diff --git a/src/kimp/kimp.py b/src/kimp/kimp.py
index 9a087c6..a0fb08b 100644
--- a/src/kimp/kimp.py
+++ b/src/kimp/kimp.py
@@ -73,7 +73,7 @@ def same_loop(self, c1: CTerm, c2: CTerm) -> bool:
     def can_make_custom_step(self, c: CTerm) -> bool:
         return False
 
-    def custom_step(self, c: CTerm) -> KCFGExtendResult | None:
+    def custom_step(self, c: CTerm, cs: CTermSymbolic) -> KCFGExtendResult | None:
         return None
 
     def is_mergeable(self, c1: CTerm, c2: CTerm) -> bool: