diff --git a/src/vm.c b/src/vm.c
index f39cfa033..294b0d1ca 100644
--- a/src/vm.c
+++ b/src/vm.c
@@ -180,7 +180,7 @@ 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;