diff --git a/src/object.c b/src/object.c
index b899427..b6553a8 100644
--- a/src/object.c
+++ b/src/object.c
@@ -35,10 +35,6 @@ void* scalloc(size_t size, size_t count)
  */
 int listLength(const Object* listObj)
 {
-    if (!listObj || !isListy(*listObj)) {
-        return -1;
-    }
-
     int len = 0;
     FOR_POINTER_IN_LIST(listObj) {
         len++;
diff --git a/src/plfunc.c b/src/plfunc.c
index bf12a31..3762c6a 100644
--- a/src/plfunc.c
+++ b/src/plfunc.c
@@ -307,10 +307,10 @@ Object len(Object* params, int length, struct Environment* env)
 {
     Object obj1 = params[0];
 
-    Object o = numberObject(listLength(&obj1));
-    if (o.number < 0) {
+    if (!isListy(obj1)) {
         return errorObject(NOT_A_LIST);
     }
+    Object o = numberObject(listLength(&obj1));
     return o;
 }