diff --git a/src/var.c b/src/var.c
index 80e61f76..a27fa0f3 100644
--- a/src/var.c
+++ b/src/var.c
@@ -5,7 +5,7 @@
 #define UNREACHABLE (assert(false))
 
 struct pic_var *
-pic_var_new(pic_state *pic, pic_value init, struct pic_proc *conv)
+pic_var_new(pic_state *pic, pic_value init, struct pic_proc *conv /* = NULL */)
 {
   struct pic_var *var;