diff --git a/object.c b/object.c
index 61a485047e..0ab93b7a71 100644
--- a/object.c
+++ b/object.c
@@ -363,7 +363,7 @@ rb_obj_copy_ivar(VALUE dest, VALUE obj)
         dest_buf = ROBJECT_FIELDS(dest);
     }
 
-    rb_shape_copy_fields(dest, dest_buf, dest_shape_id, obj, src_buf, src_shape_id);
+    rb_shape_copy_fields(dest, dest_buf, dest_shape_id, src_buf, src_shape_id);
     rb_obj_set_shape_id(dest, dest_shape_id);
 }
 
diff --git a/shape.c b/shape.c
index f799cdf11b..b769aea78b 100644
--- a/shape.c
+++ b/shape.c
@@ -1136,7 +1136,7 @@ rb_shape_rebuild(shape_id_t initial_shape_id, shape_id_t dest_shape_id)
 }
 
 void
-rb_shape_copy_fields(VALUE dest, VALUE *dest_buf, shape_id_t dest_shape_id, VALUE src, VALUE *src_buf, shape_id_t src_shape_id)
+rb_shape_copy_fields(VALUE dest, VALUE *dest_buf, shape_id_t dest_shape_id, VALUE *src_buf, shape_id_t src_shape_id)
 {
     rb_shape_t *dest_shape = RSHAPE(dest_shape_id);
     rb_shape_t *src_shape = RSHAPE(src_shape_id);
diff --git a/shape.h b/shape.h
index eab2a08f38..4354dd9ff6 100644
--- a/shape.h
+++ b/shape.h
@@ -217,7 +217,7 @@ shape_id_t rb_shape_object_id(shape_id_t original_shape_id);
 void rb_shape_free_all(void);
 
 shape_id_t rb_shape_rebuild(shape_id_t initial_shape_id, shape_id_t dest_shape_id);
-void rb_shape_copy_fields(VALUE dest, VALUE *dest_buf, shape_id_t dest_shape_id, VALUE src, VALUE *src_buf, shape_id_t src_shape_id);
+void rb_shape_copy_fields(VALUE dest, VALUE *dest_buf, shape_id_t dest_shape_id, VALUE *src_buf, shape_id_t src_shape_id);
 void rb_shape_copy_complex_ivars(VALUE dest, VALUE obj, shape_id_t src_shape_id, st_table *fields_table);
 
 static inline bool
diff --git a/variable.c b/variable.c
index d9ef42e257..0748885bcb 100644
--- a/variable.c
+++ b/variable.c
@@ -2331,7 +2331,7 @@ rb_copy_generic_ivar(VALUE dest, VALUE obj)
         new_fields_obj = rb_imemo_fields_new(rb_obj_class(dest), RSHAPE_CAPACITY(dest_shape_id));
         VALUE *src_buf = rb_imemo_fields_ptr(fields_obj);
         VALUE *dest_buf = rb_imemo_fields_ptr(new_fields_obj);
-        rb_shape_copy_fields(new_fields_obj, dest_buf, dest_shape_id, obj, src_buf, src_shape_id);
+        rb_shape_copy_fields(new_fields_obj, dest_buf, dest_shape_id, src_buf, src_shape_id);
         RBASIC_SET_SHAPE_ID(new_fields_obj, dest_shape_id);
 
         RB_VM_LOCKING() {