diff --git a/app/lib/tag_manager.rb b/app/lib/tag_manager.rb
index c1bd2973ed1..5a6284cc5b6 100644
--- a/app/lib/tag_manager.rb
+++ b/app/lib/tag_manager.rb
@@ -18,7 +18,7 @@ class TagManager
     return if domain.nil?
 
     uri = Addressable::URI.new
-    uri.host = domain.delete_suffix('/')
+    uri.host = domain.strip.delete_suffix('/')
     uri.normalized_host
   end
 
diff --git a/spec/lib/tag_manager_spec.rb b/spec/lib/tag_manager_spec.rb
index 38203a55f70..927214bb40c 100644
--- a/spec/lib/tag_manager_spec.rb
+++ b/spec/lib/tag_manager_spec.rb
@@ -54,12 +54,44 @@ RSpec.describe TagManager do
   end
 
   describe '#normalize_domain' do
-    it 'returns nil if the given parameter is nil' do
-      expect(described_class.instance.normalize_domain(nil)).to be_nil
+    subject { described_class.instance.normalize_domain(domain) }
+
+    context 'with a nil value' do
+      let(:domain) { nil }
+
+      it { is_expected.to be_nil }
+    end
+
+    context 'with a blank value' do
+      let(:domain) { '' }
+
+      it { is_expected.to be_blank }
+    end
+
+    context 'with a mixed case string' do
+      let(:domain) { 'DoMaIn.Example.com' }
+
+      it { is_expected.to eq('domain.example.com') }
     end
 
-    it 'returns normalized domain' do
-      expect(described_class.instance.normalize_domain('DoMaIn.Example.com/')).to eq 'domain.example.com'
+    context 'with a trailing slash string' do
+      let(:domain) { 'domain.example.com/' }
+
+      it { is_expected.to eq('domain.example.com') }
+    end
+
+    context 'with a space padded string' do
+      let(:domain) { '  domain.example.com  ' }
+
+      it { is_expected.to eq('domain.example.com') }
+    end
+
+    context 'with an invalid domain string' do
+      let(:domain) { '  !@#$@#$@$@#  ' }
+
+      it 'raises invalid uri error' do
+        expect { subject }.to raise_error(Addressable::URI::InvalidURIError)
+      end
     end
   end
 
diff --git a/spec/models/instance_moderation_note_spec.rb b/spec/models/instance_moderation_note_spec.rb
index 4d77d497122..011b001cc72 100644
--- a/spec/models/instance_moderation_note_spec.rb
+++ b/spec/models/instance_moderation_note_spec.rb
@@ -5,7 +5,7 @@ require 'rails_helper'
 RSpec.describe InstanceModerationNote do
   describe 'chronological' do
     it 'returns the instance notes sorted by oldest first' do
-      instance = Instance.find_or_initialize_by(domain: TagManager.instance.normalize_domain('mastodon.example'))
+      instance = Instance.find_or_initialize_by(domain: 'mastodon.example')
 
       note1 = Fabricate(:instance_moderation_note, domain: instance.domain)
       note2 = Fabricate(:instance_moderation_note, domain: instance.domain)