diff --git a/include/picrin/pair.h b/include/picrin/pair.h
index 632e45dd..61a34599 100644
--- a/include/picrin/pair.h
+++ b/include/picrin/pair.h
@@ -22,7 +22,7 @@ pic_value pic_cons(pic_state *, pic_value, pic_value);
 pic_value pic_car(pic_state *, pic_value);
 pic_value pic_cdr(pic_state *, pic_value);
 
-bool pic_list_p(pic_state *, pic_value);
+bool pic_list_p(pic_value);
 pic_value pic_list1(pic_state *, pic_value);
 pic_value pic_list2(pic_state *, pic_value, pic_value);
 pic_value pic_list3(pic_state *, pic_value, pic_value, pic_value);
diff --git a/src/codegen.c b/src/codegen.c
index f6bf09a0..d85471ae 100644
--- a/src/codegen.c
+++ b/src/codegen.c
@@ -303,7 +303,7 @@ analyze_node(analyze_state *state, pic_value obj, bool tailpos)
   case PIC_TT_PAIR: {
     pic_value proc;
 
-    if (! pic_list_p(pic, obj)) {
+    if (! pic_list_p(obj)) {
       pic_errorf(pic, "invalid expression given: ~s", obj);
     }
 
diff --git a/src/pair.c b/src/pair.c
index a633ab17..ab6e213b 100644
--- a/src/pair.c
+++ b/src/pair.c
@@ -46,13 +46,11 @@ pic_cdr(pic_state *pic, pic_value obj)
 }
 
 bool
-pic_list_p(pic_state *pic, pic_value obj)
+pic_list_p(pic_value obj)
 {
   pic_value local, rapid;
   int i;
 
-  UNUSED(pic);
-
   /* Floyd's cycle-finding algorithm. */
 
   local = rapid = obj;
@@ -443,7 +441,7 @@ pic_pair_list_p(pic_state *pic)
 
   pic_get_args(pic, "o", &v);
 
-  return pic_bool_value(pic_list_p(pic, v));
+  return pic_bool_value(pic_list_p(v));
 }
 
 static pic_value
diff --git a/src/vm.c b/src/vm.c
index e055e593..877fcf29 100644
--- a/src/vm.c
+++ b/src/vm.c
@@ -419,7 +419,7 @@ pic_apply(pic_state *pic, struct pic_proc *proc, pic_value argv)
     goto L_RAISE;
   }
 
-  if (! pic_list_p(pic, argv)) {
+  if (! pic_list_p(argv)) {
     pic_error(pic, "argv must be a proper list");
   }