diff --git a/src/protocol/mod.rs b/src/protocol/mod.rs
index 64db29608cbe90b1fc3e2601f345642dbea7b1be..bdf9a9cb2b715ca4b6dc13ab5267ff845deac23b 100644
--- a/src/protocol/mod.rs
+++ b/src/protocol/mod.rs
@@ -272,8 +272,19 @@ impl ProtocolDescription {
             },
             CTP::Input => if let Value::Input(_) = argument { true } else { false },
             CTP::Output => if let Value::Output(_) = argument { true } else { false },
-            CTP::Instance(_definition_id, _num_embedded) => {
-                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 {
+                    DefinedTypeVariant::Enum(definition) => {
+                        if let Value::Enum(variant_value) = argument {
+                            let is_valid = definition.variants.iter()
+                                .any(|v| v.value == *variant_value);
+                            return is_valid;
+                        }
+                    },
+                    _ => todo!("implement full type checking on user-supplied arguments"),
+                }
+
                 return false;
             },
         }