diff --git a/rack-protection/lib/rack/protection/content_security_policy.rb b/rack-protection/lib/rack/protection/content_security_policy.rb
index 400288e79d..19f64315b4 100644
--- a/rack-protection/lib/rack/protection/content_security_policy.rb
+++ b/rack-protection/lib/rack/protection/content_security_policy.rb
@@ -62,7 +62,7 @@ def csp_policy
         # Set these key values to boolean 'true' to include in policy
         NO_ARG_DIRECTIVES.each do |d|
           if options.key?(d) && options[d].is_a?(TrueClass)
-            directives << d.to_s.gsub(/_/, '-')
+            directives << d.to_s.tr('_', '-')
           end
         end