diff --git a/.travis-ci.sh b/.travis-ci.sh
index 9e94b594..88b97df6 100644
--- a/.travis-ci.sh
+++ b/.travis-ci.sh
@@ -1,7 +1,7 @@
 #!/bin/bash
 set -e # halt script on error
 
-HTMLPROOFER_OPTIONS="./_site --check-html --check-opengraph --report-missing-names --log-level=:debug"
+HTMLPROOFER_OPTIONS="./_site --internal-domains=pid.codes --check-html --check-opengraph --report-missing-names --log-level=:debug"
 
 bundle exec jekyll doctor
 bundle exec jekyll build
diff --git a/Gemfile b/Gemfile
index 88c9b37c..db6cec36 100644
--- a/Gemfile
+++ b/Gemfile
@@ -11,4 +11,4 @@ end
 gem "github-pages", versions['github-pages']
 gem "redcarpet"
 
-gem "html-proofer"
+gem "html-proofer", '>=3.3.1'