diff --git a/app/services/members/create_service.rb b/app/services/members/create_service.rb
index f6bad74736c..d6b17ec10be 100644
--- a/app/services/members/create_service.rb
+++ b/app/services/members/create_service.rb
@@ -23,7 +23,16 @@ module Members
 
       members.each do |member|
         if member.errors.any?
-          errors << "#{member.user.username}: #{member.errors.full_messages.to_sentence}"
+          current_error =
+            # Invited users may not have an associated user
+            if member.user.present?
+              "#{member.user.username}: "
+            else
+              ""
+            end
+
+          current_error += member.errors.full_messages.to_sentence
+          errors << current_error
         else
           after_execute(member: member)
         end
diff --git a/changelogs/unreleased/sh-fix-invited-members.yml b/changelogs/unreleased/sh-fix-invited-members.yml
new file mode 100644
index 00000000000..96e43e1aa53
--- /dev/null
+++ b/changelogs/unreleased/sh-fix-invited-members.yml
@@ -0,0 +1,5 @@
+---
+title: Fix Error 500 when inviting user already present
+merge_request: 28198
+author:
+type: fixed
diff --git a/spec/services/members/create_service_spec.rb b/spec/services/members/create_service_spec.rb
index 2c4fb131ed9..674fe0f666e 100644
--- a/spec/services/members/create_service_spec.rb
+++ b/spec/services/members/create_service_spec.rb
@@ -44,7 +44,18 @@ describe Members::CreateService do
     result = described_class.new(user, params).execute(project)
 
     expect(result[:status]).to eq(:error)
-    expect(result[:message]).to include(project_user.username)
+    expect(result[:message]).to include("#{project_user.username}: Access level is not included in the list")
     expect(project.users).not_to include project_user
   end
+
+  it 'does not add a member with an existing invite' do
+    invited_member = create(:project_member, :invited, project: project)
+
+    params = { user_ids: invited_member.invite_email,
+               access_level: Gitlab::Access::GUEST }
+    result = described_class.new(user, params).execute(project)
+
+    expect(result[:status]).to eq(:error)
+    expect(result[:message]).to eq('Invite email has already been taken')
+  end
 end