diff --git a/src/ast.rs b/src/ast.rs
index ed59891..3e9026b 100644
--- a/src/ast.rs
+++ b/src/ast.rs
@@ -225,6 +225,9 @@ impl Function
 {
 	pub fn new(declaration: &std::rc::Rc<FunctionDeclaration>, arguments: Vec<Box<Term>>) -> Self
 	{
+		assert_eq!(declaration.arity, arguments.len(),
+			"function has a different number of arguments then declared");
+
 		Self
 		{
 			declaration: std::rc::Rc::clone(declaration),
@@ -377,6 +380,9 @@ impl Predicate
 {
 	pub fn new(declaration: &std::rc::Rc<PredicateDeclaration>, arguments: Vec<Box<Term>>) -> Self
 	{
+		assert_eq!(declaration.arity, arguments.len(),
+			"predicate has a different number of arguments then declared");
+
 		Self
 		{
 			declaration: std::rc::Rc::clone(declaration),