diff --git a/app/models/account.rb b/app/models/account.rb
index f7a7f4a7b4..2c2840ee48 100644
--- a/app/models/account.rb
+++ b/app/models/account.rb
@@ -58,8 +58,9 @@ class Account < ApplicationRecord
     hub_url
   )
 
-  USERNAME_RE = /[a-z0-9_]+([a-z0-9_\.-]+[a-z0-9_]+)?/i
-  MENTION_RE  = /(?<=^|[^\/[:word:]])@((#{USERNAME_RE})(?:@[[:word:]\.\-]+[a-z0-9]+)?)/i
+  USERNAME_RE   = /[a-z0-9_]+([a-z0-9_\.-]+[a-z0-9_]+)?/i
+  MENTION_RE    = /(?<=^|[^\/[:word:]])@((#{USERNAME_RE})(?:@[[:word:]\.\-]+[a-z0-9]+)?)/i
+  URL_PREFIX_RE = /\Ahttp(s?):\/\/[^\/]+/
 
   include AccountAssociations
   include AccountAvatar
@@ -379,7 +380,7 @@ class Account < ApplicationRecord
   def synchronization_uri_prefix
     return 'local' if local?
 
-    @synchronization_uri_prefix ||= uri[/http(s?):\/\/[^\/]+\//]
+    @synchronization_uri_prefix ||= "#{uri[URL_PREFIX_RE]}/"
   end
 
   class Field < ActiveModelSerializers::Model
diff --git a/app/models/concerns/account_interactions.rb b/app/models/concerns/account_interactions.rb
index 4bf62539ce..8f19176a74 100644
--- a/app/models/concerns/account_interactions.rb
+++ b/app/models/concerns/account_interactions.rb
@@ -254,10 +254,13 @@ module AccountInteractions
          .where('users.current_sign_in_at > ?', User::ACTIVE_DURATION.ago)
   end
 
-  def remote_followers_hash(url_prefix)
-    Rails.cache.fetch("followers_hash:#{id}:#{url_prefix}") do
+  def remote_followers_hash(url)
+    url_prefix = url[Account::URL_PREFIX_RE]
+    return if url_prefix.blank?
+
+    Rails.cache.fetch("followers_hash:#{id}:#{url_prefix}/") do
       digest = "\x00" * 32
-      followers.where(Account.arel_table[:uri].matches(url_prefix + '%', false, true)).pluck_each(:uri) do |uri|
+      followers.where(Account.arel_table[:uri].matches("#{Account.sanitize_sql_like(url_prefix)}/%", false, true)).or(followers.where(uri: url_prefix)).pluck_each(:uri) do |uri|
         Xorcist.xor!(digest, Digest::SHA256.digest(uri))
       end
       digest.unpack('H*')[0]
diff --git a/app/workers/activitypub/delivery_worker.rb b/app/workers/activitypub/delivery_worker.rb
index 6c5a576a70..788f2cf809 100644
--- a/app/workers/activitypub/delivery_worker.rb
+++ b/app/workers/activitypub/delivery_worker.rb
@@ -44,11 +44,7 @@ class ActivityPub::DeliveryWorker
   end
 
   def synchronization_header
-    "collectionId=\"#{account_followers_url(@source_account)}\", digest=\"#{@source_account.remote_followers_hash(inbox_url_prefix)}\", url=\"#{account_followers_synchronization_url(@source_account)}\""
-  end
-
-  def inbox_url_prefix
-    @inbox_url[/http(s?):\/\/[^\/]+\//]
+    "collectionId=\"#{account_followers_url(@source_account)}\", digest=\"#{@source_account.remote_followers_hash(@inbox_url)}\", url=\"#{account_followers_synchronization_url(@source_account)}\""
   end
 
   def perform_request
diff --git a/spec/models/concerns/account_interactions_spec.rb b/spec/models/concerns/account_interactions_spec.rb
index 85fbf7e79c..ca243ebc5e 100644
--- a/spec/models/concerns/account_interactions_spec.rb
+++ b/spec/models/concerns/account_interactions_spec.rb
@@ -539,46 +539,57 @@ describe AccountInteractions do
     end
   end
 
-  describe '#followers_hash' do
+  describe '#remote_followers_hash' do
     let(:me) { Fabricate(:account, username: 'Me') }
     let(:remote_1) { Fabricate(:account, username: 'alice', domain: 'example.org', uri: 'https://example.org/users/alice') }
     let(:remote_2) { Fabricate(:account, username: 'bob', domain: 'example.org', uri: 'https://example.org/users/bob') }
-    let(:remote_3) { Fabricate(:account, username: 'eve', domain: 'foo.org', uri: 'https://foo.org/users/eve') }
+    let(:remote_3) { Fabricate(:account, username: 'instance-actor', domain: 'example.org', uri: 'https://example.org') }
+    let(:remote_4) { Fabricate(:account, username: 'eve', domain: 'foo.org', uri: 'https://foo.org/users/eve') }
 
     before do
       remote_1.follow!(me)
       remote_2.follow!(me)
       remote_3.follow!(me)
+      remote_4.follow!(me)
       me.follow!(remote_1)
     end
 
-    context 'on a local user' do
-      it 'returns correct hash for remote domains' do
-        expect(me.remote_followers_hash('https://example.org/')).to eq '707962e297b7bd94468a21bc8e506a1bcea607a9142cd64e27c9b106b2a5f6ec'
-        expect(me.remote_followers_hash('https://foo.org/')).to eq 'ccb9c18a67134cfff9d62c7f7e7eb88e6b803446c244b84265565f4eba29df0e'
-      end
-
-      it 'invalidates cache as needed when removing or adding followers' do
-        expect(me.remote_followers_hash('https://example.org/')).to eq '707962e297b7bd94468a21bc8e506a1bcea607a9142cd64e27c9b106b2a5f6ec'
-        remote_1.unfollow!(me)
-        expect(me.remote_followers_hash('https://example.org/')).to eq '241b00794ce9b46aa864f3220afadef128318da2659782985bac5ed5bd436bff'
-        remote_1.follow!(me)
-        expect(me.remote_followers_hash('https://example.org/')).to eq '707962e297b7bd94468a21bc8e506a1bcea607a9142cd64e27c9b106b2a5f6ec'
-      end
+    it 'returns correct hash for remote domains' do
+      expect(me.remote_followers_hash('https://example.org/')).to eq '20aecbe774b3d61c25094370baf370012b9271c5b172ecedb05caff8d79ef0c7'
+      expect(me.remote_followers_hash('https://foo.org/')).to eq 'ccb9c18a67134cfff9d62c7f7e7eb88e6b803446c244b84265565f4eba29df0e'
+      expect(me.remote_followers_hash('https://foo.org.evil.com/')).to eq '0000000000000000000000000000000000000000000000000000000000000000'
+      expect(me.remote_followers_hash('https://foo')).to eq '0000000000000000000000000000000000000000000000000000000000000000'
     end
 
-    context 'on a remote user' do
-      it 'returns correct hash for remote domains' do
-        expect(remote_1.local_followers_hash).to eq Digest::SHA256.hexdigest(ActivityPub::TagManager.instance.uri_for(me))
-      end
+    it 'invalidates cache as needed when removing or adding followers' do
+      expect(me.remote_followers_hash('https://example.org/')).to eq '20aecbe774b3d61c25094370baf370012b9271c5b172ecedb05caff8d79ef0c7'
+      remote_3.unfollow!(me)
+      expect(me.remote_followers_hash('https://example.org/')).to eq '707962e297b7bd94468a21bc8e506a1bcea607a9142cd64e27c9b106b2a5f6ec'
+      remote_1.unfollow!(me)
+      expect(me.remote_followers_hash('https://example.org/')).to eq '241b00794ce9b46aa864f3220afadef128318da2659782985bac5ed5bd436bff'
+      remote_1.follow!(me)
+      expect(me.remote_followers_hash('https://example.org/')).to eq '707962e297b7bd94468a21bc8e506a1bcea607a9142cd64e27c9b106b2a5f6ec'
+    end
+  end
 
-      it 'invalidates cache as needed when removing or adding followers' do
-        expect(remote_1.local_followers_hash).to eq Digest::SHA256.hexdigest(ActivityPub::TagManager.instance.uri_for(me))
-        me.unfollow!(remote_1)
-        expect(remote_1.local_followers_hash).to eq '0000000000000000000000000000000000000000000000000000000000000000'
-        me.follow!(remote_1)
-        expect(remote_1.local_followers_hash).to eq Digest::SHA256.hexdigest(ActivityPub::TagManager.instance.uri_for(me))
-      end
+  describe '#local_followers_hash' do
+    let(:me) { Fabricate(:account, username: 'Me') }
+    let(:remote_1) { Fabricate(:account, username: 'alice', domain: 'example.org', uri: 'https://example.org/users/alice') }
+
+    before do
+      me.follow!(remote_1)
+    end
+
+    it 'returns correct hash for local users' do
+      expect(remote_1.local_followers_hash).to eq Digest::SHA256.hexdigest(ActivityPub::TagManager.instance.uri_for(me))
+    end
+
+    it 'invalidates cache as needed when removing or adding followers' do
+      expect(remote_1.local_followers_hash).to eq Digest::SHA256.hexdigest(ActivityPub::TagManager.instance.uri_for(me))
+      me.unfollow!(remote_1)
+      expect(remote_1.local_followers_hash).to eq '0000000000000000000000000000000000000000000000000000000000000000'
+      me.follow!(remote_1)
+      expect(remote_1.local_followers_hash).to eq Digest::SHA256.hexdigest(ActivityPub::TagManager.instance.uri_for(me))
     end
   end
 
diff --git a/spec/workers/activitypub/delivery_worker_spec.rb b/spec/workers/activitypub/delivery_worker_spec.rb
index f4633731e5..d39393d507 100644
--- a/spec/workers/activitypub/delivery_worker_spec.rb
+++ b/spec/workers/activitypub/delivery_worker_spec.rb
@@ -11,7 +11,7 @@ describe ActivityPub::DeliveryWorker do
   let(:payload) { 'test' }
 
   before do
-    allow_any_instance_of(Account).to receive(:remote_followers_hash).with('https://example.com/').and_return('somehash')
+    allow_any_instance_of(Account).to receive(:remote_followers_hash).with('https://example.com/api').and_return('somehash')
   end
 
   describe 'perform' do