diff --git a/Library/Homebrew/extend/io.rb b/Library/Homebrew/extend/io.rb
index 55db17651e..a6eb7d4b70 100644
--- a/Library/Homebrew/extend/io.rb
+++ b/Library/Homebrew/extend/io.rb
@@ -2,7 +2,7 @@
 # frozen_string_literal: true
 
 class IO
-  sig { params(sep: BasicObject).returns(String) }
+  sig { params(sep: String).returns(String) }
   def readline_nonblock(sep = $INPUT_RECORD_SEPARATOR)
     line = +""
     buffer = +""