diff --git a/lib/html/pipeline/sanitization_filter.rb b/lib/html/pipeline/sanitization_filter.rb
index b7e537f7..6d463e9f 100644
--- a/lib/html/pipeline/sanitization_filter.rb
+++ b/lib/html/pipeline/sanitization_filter.rb
@@ -73,7 +73,7 @@ class SanitizationFilter < Filter
vspace width itemprop].freeze
}.freeze,
protocols: {
- 'a' => { 'href' => ANCHOR_SCHEMES },
+ 'a' => { 'href' => ANCHOR_SCHEMES }.freeze,
'blockquote' => { 'cite' => ['http', 'https', :relative].freeze },
'del' => { 'cite' => ['http', 'https', :relative].freeze },
'ins' => { 'cite' => ['http', 'https', :relative].freeze },