diff --git a/test/test_error_highlight.rb b/test/test_error_highlight.rb
index 09f29dc..6136bcd 100644
--- a/test/test_error_highlight.rb
+++ b/test/test_error_highlight.rb
@@ -1362,12 +1362,17 @@ def test_errors_on_terminal_window_smaller_than_min_width
 
     warning = nil
     original_warn = Warning.method(:warn)
-    Warning.define_singleton_method(:warn) {|s| warning = s}
+    Warning.class_eval do
+      remove_method(:warn)
+      define_method(:warn) {|str| warning = str}
+    end
     begin
       ErrorHighlight::DefaultFormatter.max_snippet_width = custom_max_width
     ensure
-      Warning.singleton_class.remove_method(:warn)
-      Warning.define_singleton_method(:warn, original_warn)
+      Warning.class_eval do
+        remove_method(:warn)
+        define_method(:warn, original_warn)
+      end
     end
     assert_match "'max_snippet_width' adjusted to minimum value of #{min_snippet_width}", warning