diff --git a/src/vm.c b/src/vm.c
index ab07f5718..c9e80e0ed 100644
--- a/src/vm.c
+++ b/src/vm.c
@@ -180,16 +180,16 @@ uvenv(mrb_state *mrb, int up)
   return e;
 }
 
-static inline int
+static inline mrb_bool
 is_strict(mrb_state *mrb, struct REnv *e)
 {
   int cioff = e->cioff;
 
   if (cioff >= 0 && mrb->c->cibase[cioff].proc &&
       MRB_PROC_STRICT_P(mrb->c->cibase[cioff].proc)) {
-    return 1;
+    return TRUE;
   }
-  return 0;
+  return FALSE;
 }
 
 static inline struct REnv*