diff --git a/include/picrin/value.h b/include/picrin/value.h
index ef763a0a..88266491 100644
--- a/include/picrin/value.h
+++ b/include/picrin/value.h
@@ -10,13 +10,13 @@ extern "C" {
 #endif
 
 /**
- * pic_sym is just an alias to int.
+ * pic_sym is just an alias to unsigned int.
  * the value 0 for pic_sym is guaranteed to resolve to no symbol.
  * if you are defining a function that returns optional<pic_sym>,
  * the zero symbol would be useful for such situation.
  */
 
-typedef int pic_sym;
+typedef unsigned pic_sym;
 
 /**
  * `undef` values never seen from user-end: that is,