diff --git a/app/models/user_email.rb b/app/models/user_email.rb
index 101eddcd5c5..4a5a9e80cab 100644
--- a/app/models/user_email.rb
+++ b/app/models/user_email.rb
@@ -11,7 +11,7 @@ class UserEmail < ActiveRecord::Base
   validates :email, email: true, format: { with: EmailValidator.email_regex },
                     if: :validate_email?
 
-  validates :primary, uniqueness: { scope: [:user_id] }, if: :user_id
+  validates :primary, uniqueness: { scope: [:user_id] }, if: [:user_id, :primary]
   validate :user_id_not_changed, if: :primary
   validate :unique_email
 
diff --git a/db/migrate/20171220181249_change_user_emails_primary_index.rb b/db/migrate/20171220181249_change_user_emails_primary_index.rb
new file mode 100644
index 00000000000..f78a5d77897
--- /dev/null
+++ b/db/migrate/20171220181249_change_user_emails_primary_index.rb
@@ -0,0 +1,11 @@
+class ChangeUserEmailsPrimaryIndex < ActiveRecord::Migration[5.1]
+  def up
+    remove_index :user_emails, [:user_id, :primary]
+    add_index :user_emails, [:user_id, :primary], unique: true, where: '"primary"'
+  end
+
+  def down
+    remove_index :user_emails, [:user_id, :primary]
+    add_index :user_emails, [:user_id, :primary], unique: true
+  end
+end
diff --git a/spec/models/user_email_spec.rb b/spec/models/user_email_spec.rb
new file mode 100644
index 00000000000..c2d822491ea
--- /dev/null
+++ b/spec/models/user_email_spec.rb
@@ -0,0 +1,36 @@
+require 'rails_helper'
+require_dependency 'user_email'
+
+describe UserEmail do
+  context "validation" do
+    it "allows only one primary email" do
+      user = Fabricate(:user_single_email)
+      expect {
+        Fabricate(:alternate_email, user: user, primary: true)
+      }.to raise_error(ActiveRecord::RecordInvalid)
+    end
+
+    it "allows multiple secondary emails" do
+      user = Fabricate(:user_single_email)
+      Fabricate(:alternate_email, user: user, primary: false)
+      Fabricate(:alternate_email, user: user, primary: false)
+      expect(user.user_emails.count).to eq 3
+    end
+  end
+
+  context "indexes" do
+    it "allows only one primary email" do
+      user = Fabricate(:user_single_email)
+      expect {
+        Fabricate.build(:alternate_email, user: user, primary: true).save(validate: false)
+      }.to raise_error(ActiveRecord::RecordNotUnique)
+    end
+
+    it "allows multiple secondary emails" do
+      user = Fabricate(:user_single_email)
+      Fabricate.build(:alternate_email, user: user, primary: false).save(validate: false)
+      Fabricate.build(:alternate_email, user: user, primary: false).save(validate: false)
+      expect(user.user_emails.count).to eq 3
+    end
+  end
+end