diff --git a/src/syntax.c b/src/syntax.c
index fc94535..9de72bf 100644
--- a/src/syntax.c
+++ b/src/syntax.c
@@ -46,7 +46,7 @@ struct Object *syntax_define(
     struct Object *const environment
 ) {
     assert(Object_is_pair(environment));
-    assert(OBJECT_IS_NULL(environment->pair.cdr));
+    assert(OBJECT_IS_LIST_HEAD(environment->pair.cdr));
     assert(OBJECT_IS_LIST_HEAD(environment->pair.car));
 
     assert(Object_is_pair(args));
diff --git a/tests/syntax.scm b/tests/syntax.scm
index 973a40e..24b691e 100644
--- a/tests/syntax.scm
+++ b/tests/syntax.scm
@@ -24,6 +24,13 @@
     (define (sum a b) (+ a b))
     (define (inc a)   (+ a 1))
     (sum (inc 1) (inc 2))))
+(assert-equal
+  11
+  (begin
+    (define (make-inc const-num) (lambda (var-num) (+ var-num const-num)))
+    (define inc1  (make-inc 1))
+    (define inc10 (make-inc 10))
+    (inc1 (inc10 0))))
 
 ;;; if ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
 (assert-equal 123 (if #true 123 456))