diff --git a/manifests/init.pp b/manifests/init.pp
index b51b697..10f49d3 100644
--- a/manifests/init.pp
+++ b/manifests/init.pp
@@ -90,7 +90,9 @@
   $ini_settings = {
     'server'              => { setting => 'server', value => $server,},
     'ca_server'           => { setting => 'ca_server', value => $ca_server,},
-    'certname'            => { setting => 'certname', value => $certname,},
+    # certname must be lower case
+    # see https://puppet.com/docs/puppet/latest/configuration.html#certname and https://tickets.puppetlabs.com/browse/PUP-2551
+    'certname'            => { setting => 'certname', value => downcase($certname),},
     'environment'         => { setting => 'environment', value => $env,},
     'trusted_node_data'   => { setting => 'trusted_node_data', value => true,},
     'graph'               => { setting => 'graph', value => $graph,},
diff --git a/spec/classes/init_spec.rb b/spec/classes/init_spec.rb
index 3259751..a0949c2 100644
--- a/spec/classes/init_spec.rb
+++ b/spec/classes/init_spec.rb
@@ -251,6 +251,21 @@
     end
   end
 
+  describe 'with certname containing capitalization' do
+    let(:params) { { :certname => 'puppet.example.COM' } }
+
+    it do
+      should contain_ini_setting('certname').with({
+        :ensure  => 'present',
+        :setting => 'certname',
+        :value   => 'puppet.example.com',
+        :path    => '/etc/puppetlabs/puppet/puppet.conf',
+        :section => 'main',
+        :require => 'File[puppet_config]',
+      })
+    end
+  end
+
   describe 'with custom_settings specified' do
     let(:params) { {
       :custom_settings => {