diff --git a/src/interp.rs b/src/interp.rs
index b5a9e33..e0617bf 100644
--- a/src/interp.rs
+++ b/src/interp.rs
@@ -949,6 +949,7 @@ pub fn const_eval(
                 ConstVal::None
             })
         }
+        (_, args) if args.iter().any(|&arg| arg == ConstVal::None) => None,
         (op, args) => unimplemented!(
             "Undefined operator or arg combination: {:?}, {:?}",
             op,