diff --git a/app/helpers/tree_helper.rb b/app/helpers/tree_helper.rb
index 5e7c20ef51e..dc42caa70e5 100644
--- a/app/helpers/tree_helper.rb
+++ b/app/helpers/tree_helper.rb
@@ -90,7 +90,7 @@ module TreeHelper
   end
 
   def commit_in_single_accessible_branch
-    branch_name = html_escape(selected_branch)
+    branch_name = ERB::Util.html_escape(selected_branch)
 
     message = _("Your changes can be committed to %{branch_name} because a merge "\
                 "request is open.") % { branch_name: "<strong>#{branch_name}</strong>" }
diff --git a/changelogs/unreleased/45436-markdown-is-not-rendering-error-loading-viewer-undefined-method-html_escape.yml b/changelogs/unreleased/45436-markdown-is-not-rendering-error-loading-viewer-undefined-method-html_escape.yml
new file mode 100644
index 00000000000..0f1d111ca58
--- /dev/null
+++ b/changelogs/unreleased/45436-markdown-is-not-rendering-error-loading-viewer-undefined-method-html_escape.yml
@@ -0,0 +1,5 @@
+---
+title: Fix undefined `html_escape` method during markdown rendering
+merge_request: 18418
+author:
+type: fixed
diff --git a/lib/banzai/renderer/common_mark/html.rb b/lib/banzai/renderer/common_mark/html.rb
index c7a54629f31..46b609c36b0 100644
--- a/lib/banzai/renderer/common_mark/html.rb
+++ b/lib/banzai/renderer/common_mark/html.rb
@@ -9,7 +9,7 @@ module Banzai
             lang_attr = lang.present? ? %Q{ lang="#{lang}"} : ''
             result    =
               "<pre>" \
-                "<code#{lang_attr}>#{html_escape(code)}</code>" \
+                "<code#{lang_attr}>#{ERB::Util.html_escape(code)}</code>" \
               "</pre>"
 
             out(result)
diff --git a/lib/banzai/renderer/redcarpet/html.rb b/lib/banzai/renderer/redcarpet/html.rb
index 94df5d8b1e1..30e815f1224 100644
--- a/lib/banzai/renderer/redcarpet/html.rb
+++ b/lib/banzai/renderer/redcarpet/html.rb
@@ -6,7 +6,7 @@ module Banzai
           lang_attr = lang ? %Q{ lang="#{lang}"} : ''
 
           "\n<pre>" \
-            "<code#{lang_attr}>#{html_escape(code)}</code>" \
+            "<code#{lang_attr}>#{ERB::Util.html_escape(code)}</code>" \
           "</pre>"
         end
       end