diff --git a/core/builtin.rbs b/core/builtin.rbs
index 2b1a0f71a..b9d53f009 100644
--- a/core/builtin.rbs
+++ b/core/builtin.rbs
@@ -23,7 +23,7 @@ interface _ToHash[K, V]
 end
 
 interface _ToProc
-  def to_proc: () -> untyped
+  def to_proc: () -> Proc
 end
 
 interface _ToPath