diff --git a/lib/rubocop/config_validator.rb b/lib/rubocop/config_validator.rb
index 5a28ee6ec2b..8f0aece3fab 100644
--- a/lib/rubocop/config_validator.rb
+++ b/lib/rubocop/config_validator.rb
@@ -131,7 +131,9 @@ def validate_parameter_names(valid_cop_names)
           next if COMMON_PARAMS.include?(param) || default_config.key?(param)
 
           supported_params = default_config.keys - INTERNAL_PARAMS
-          warn Rainbow(<<~MESSAGE).yellow
+          # FIXME: Remove .to_s, which works around a JRuby bug:
+          #   https://github.com/jruby/jruby/issues/5935
+          warn Rainbow(<<~MESSAGE).yellow.to_s
             Warning: #{name} does not support #{param} parameter.
 
             Supported parameters are: