diff --git a/ractor.rb b/ractor.rb
index 3119682dd1..39dc8c9dc1 100644
--- a/ractor.rb
+++ b/ractor.rb
@@ -884,7 +884,7 @@ class Ractor
   # return true if the current ractor is main ractor
   def self.main?
     __builtin_cexpr! %q{
-      GET_VM()->ractor.main_ractor == rb_ec_ractor_ptr(ec)
+      RBOOL(GET_VM()->ractor.main_ractor == rb_ec_ractor_ptr(ec))
     }
   end