diff --git a/src/protocol/mod.rs b/src/protocol/mod.rs
index f9d1295140a2ace7cbcc910ac216758e5510aed5..8c3b36972848a66314c30f688e57cd46b27c4434 100644
--- a/src/protocol/mod.rs
+++ b/src/protocol/mod.rs
@@ -187,6 +187,7 @@ impl ProtocolDescription {
             },
             CTP::Input => if let Value::Input(_) = argument { true } else { false },
             CTP::Output => if let Value::Output(_) = argument { true } else { false },
+            CTP::Tuple(_) => todo!("implement full type checking on user-supplied arguments"),
             CTP::Instance(definition_id, _num_embedded) => {
                 let definition = self.types.get_base_definition(definition_id).unwrap();
                 match &definition.definition {