diff --git a/include/picrin.h b/include/picrin.h
index 17c7a7d4..189a295b 100644
--- a/include/picrin.h
+++ b/include/picrin.h
@@ -103,6 +103,7 @@ pic_value pic_str_new_cstr(pic_state *, const char *);
 
 struct pic_vector *pic_vec_new(pic_state *, size_t);
 struct pic_vector *pic_vec_new_from_list(pic_state *, pic_value);
+void pic_vec_extend_ip(pic_state *, struct pic_vector *, int);
 
 int pic_parse_file(pic_state *, FILE *file, pic_value *);
 int pic_parse_cstr(pic_state *, const char *, pic_value *);
diff --git a/src/vector.c b/src/vector.c
index 9e695647..e4137680 100644
--- a/src/vector.c
+++ b/src/vector.c
@@ -32,6 +32,19 @@ pic_vec_new_from_list(pic_state *pic, pic_value data)
   return vec;
 }
 
+void
+pic_vec_extend_ip(pic_state *pic, struct pic_vector *vec, int size)
+{
+  int len, i;
+
+  len = vec->len;
+  vec->len = size;
+  vec->data = (pic_value *)pic_realloc(pic, vec->data, sizeof(pic_value) * size);
+  for (i = len; i < size; ++i) {
+    vec->data[i] = pic_false_value();
+  }
+}
+
 static pic_value
 pic_vec_vector_p(pic_state *pic)
 {