diff --git a/src/mapper/flattener.py b/src/mapper/flattener.py
index 40f35c0ce92f60aba8ba05fc80c73fe4e2dede6c..41eaa40d0e50c7e8916e7a77b921a19677c7515c 100644
--- a/src/mapper/flattener.py
+++ b/src/mapper/flattener.py
@@ -11,7 +11,9 @@ class FlattenMapper(IdentityMapper):
     def map_product(self, expr):
         from pymbolic.primitives import flattened_product
         return flattened_product(expr.children)
-            
+
+    def handle_unsupported_expression(self, expr):
+        return expr