diff --git a/README.md b/README.md
index ba8bf17..37cfb50 100644
--- a/README.md
+++ b/README.md
@@ -9,6 +9,9 @@ A Puppet Face and plugin built on the [YARD Documentation Tool](http://yardoc.or
 Installation
 ------------
 
+So far, this module has been developed against Puppet 3.6.x.
+It will not work with earlier versions.
+
 Currently, just `git clone` directly into the Puppet `modulepath`.
 Ensure the `yard` and `rgen` gems are installed.
 If running Ruby 1.8.7, ensure the `backports` gem is installed.
diff --git a/lib/puppet/face/yardoc.rb b/lib/puppet/face/yardoc.rb
index e6f1098..f5809b6 100644
--- a/lib/puppet/face/yardoc.rb
+++ b/lib/puppet/face/yardoc.rb
@@ -74,6 +74,9 @@ Puppet::Face.define(:yardoc, '0.0.1') do
       require 'puppetx/yardoc/yard/plugin'
       opts = args.pop
 
+      # NOTE: The retrun value of the `module` Face seems to have changed in
+      # 3.6.x. This part of the code will blow up if run under an earlier
+      # version of Puppet.
       modules = Puppet::Face[:module, :current].list
       module_list = modules[:modules_by_path].values.flatten
 
diff --git a/metadata.json b/metadata.json
index cecd0eb..f9edfc9 100644
--- a/metadata.json
+++ b/metadata.json
@@ -7,6 +7,12 @@
   "source": "",
   "project_page": null,
   "issues_url": null,
+  "requirements": [
+    {
+      "name": "puppet",
+      "version_requirement": ">= 3.6.0"
+    }
+  ],
   "dependencies": [ ]
 }