diff --git a/changelogs/unreleased/sh-fix-error-500-updating-wikis.yml b/changelogs/unreleased/sh-fix-error-500-updating-wikis.yml
new file mode 100644
index 00000000000..cf7ca05914f
--- /dev/null
+++ b/changelogs/unreleased/sh-fix-error-500-updating-wikis.yml
@@ -0,0 +1,5 @@
+---
+title: Fix Error 500s due to encoding issues when Wiki hooks fire
+merge_request:
+author:
+type: fixed
diff --git a/lib/gitlab/encoding_helper.rb b/lib/gitlab/encoding_helper.rb
index d1fd5dfe0cb..0f336fbaa10 100644
--- a/lib/gitlab/encoding_helper.rb
+++ b/lib/gitlab/encoding_helper.rb
@@ -75,7 +75,7 @@ module Gitlab
     end
 
     def binary_stringio(str)
-      StringIO.new(str || '').tap { |io| io.set_encoding(Encoding::ASCII_8BIT) }
+      StringIO.new(str.freeze || '').tap { |io| io.set_encoding(Encoding::ASCII_8BIT) }
     end
 
     private
diff --git a/spec/lib/gitlab/encoding_helper_spec.rb b/spec/lib/gitlab/encoding_helper_spec.rb
index e68c9850f6b..a5bf2f2b3df 100644
--- a/spec/lib/gitlab/encoding_helper_spec.rb
+++ b/spec/lib/gitlab/encoding_helper_spec.rb
@@ -1,3 +1,4 @@
+# coding: utf-8
 require "spec_helper"
 
 describe Gitlab::EncodingHelper do
@@ -187,4 +188,15 @@ describe Gitlab::EncodingHelper do
       end
     end
   end
+
+  describe '#binary_stringio' do
+    it 'does not mutate the original string encoding' do
+      test = 'my-test'
+
+      io_stream = ext_class.binary_stringio(test)
+
+      expect(io_stream.external_encoding.name).to eq('ASCII-8BIT')
+      expect(test.encoding.name).to eq('UTF-8')
+    end
+  end
 end