diff --git a/lib/html_pipeline/sanitization_filter.rb b/lib/html_pipeline/sanitization_filter.rb
index eaddc2cb..7edba4a5 100644
--- a/lib/html_pipeline/sanitization_filter.rb
+++ b/lib/html_pipeline/sanitization_filter.rb
@@ -1,4 +1,5 @@
 # frozen_string_literal: true
+require 'selma'
 
 class HTMLPipeline
   # A special filter with sanization routines and allowlists. This module defines