diff --git a/src/interpreter/functions.rs b/src/interpreter/functions.rs
index 5b0a881..939a971 100644
--- a/src/interpreter/functions.rs
+++ b/src/interpreter/functions.rs
@@ -47,7 +47,8 @@ impl Callable for Function {
         assert_eq!(arguments.len(), self.arity());
         let param_env = Environment::create_child(environment);
         for (arg, value) in izip!(self.params.iter(), arguments.into_iter()) {
-            param_env.borrow_mut().define(&arg, Some(value));
+            // FIXME: duplicate parameter names should be detected in the parser
+            param_env.borrow_mut().define(&arg, Some(value))?;
         }
 
         let child = Environment::create_child(&param_env);