diff --git a/CHANGELOG.md b/CHANGELOG.md
index 1c412a2..a417909 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,6 +1,7 @@
 # Changelog
 
 ## Unreleased - xxxx-xx-xx
+- Fix `validates_db_uniqueness_of` raising `NoMethodError` on PostgreSQL when the table has row-level security. PostgreSQL omits the `Key (...)=(...)` detail from the unique violation there, so the error is now matched by its index name only
 
 ## [2.1.1] - 2026-03-31
 - `lint_roller` is removed as dependency. It is not needed
diff --git a/lib/database_validations/lib/adapters/postgresql_adapter.rb b/lib/database_validations/lib/adapters/postgresql_adapter.rb
index f1fe802..0ff4fdc 100644
--- a/lib/database_validations/lib/adapters/postgresql_adapter.rb
+++ b/lib/database_validations/lib/adapters/postgresql_adapter.rb
@@ -8,8 +8,11 @@ def unique_index_name(error)
           error.message[/unique constraint "([^"]+)"/, 1]
         end
 
+        # PostgreSQL leaves the `DETAIL:  Key (...)=(...) already exists.` line out of a unique violation on a
+        # table the current role is subject to row-level security on, because the values could belong to a row the
+        # role cannot see. The columns are then unknown (nil) and the error is matched by its index name instead.
         def unique_error_columns(error)
-          error.message[/Key \((.+)\)=/, 1].split(', ')
+          error.message[/Key \((.+)\)=/, 1]&.split(', ')
         end
 
         def foreign_key_error_column(error)
diff --git a/spec/adapters/postgresql_unique_error_columns_spec.rb b/spec/adapters/postgresql_unique_error_columns_spec.rb
new file mode 100644
index 0000000..3e0f5e5
--- /dev/null
+++ b/spec/adapters/postgresql_unique_error_columns_spec.rb
@@ -0,0 +1,25 @@
+RSpec.describe 'Adapter.unique_error_columns' do
+  def build_error(message)
+    ActiveRecord::RecordNotUnique.new(message)
+  end
+
+  describe DatabaseValidations::Adapters::PostgresqlAdapter do
+    it 'extracts the columns from the error detail' do
+      error = build_error(
+        'PG::UniqueViolation: ERROR:  duplicate key value violates unique constraint "index_entities_on_a_and_b" ' \
+        'DETAIL:  Key (a, b)=(1, x) already exists.'
+      )
+
+      expect(described_class.unique_error_columns(error)).to eq(%w[a b])
+    end
+
+    it 'returns nil when the error has no detail' do
+      error = build_error(
+        'PG::UniqueViolation: ERROR:  duplicate key value violates unique constraint "index_entities_on_a_and_b"'
+      )
+
+      expect(described_class.unique_error_columns(error)).to be_nil
+      expect(described_class.unique_index_name(error)).to eq('index_entities_on_a_and_b')
+    end
+  end
+end
diff --git a/spec/validations/validates_db_uniqueness_of_row_level_security_spec.rb b/spec/validations/validates_db_uniqueness_of_row_level_security_spec.rb
new file mode 100644
index 0000000..5069493
--- /dev/null
+++ b/spec/validations/validates_db_uniqueness_of_row_level_security_spec.rb
@@ -0,0 +1,98 @@
+RSpec.describe '.validates_db_uniqueness_of with PostgreSQL row-level security' do
+  # End to end for a unique violation that carries no `DETAIL` line: see `PostgresqlAdapter.unique_error_columns`.
+  let(:role) { 'database_validations_rls_probe' }
+  let(:connection) { ActiveRecord::Base.connection }
+
+  before do
+    define_database(postgresql_configuration)
+
+    can_create_roles = connection.select_value(
+      'SELECT rolsuper OR rolcreaterole FROM pg_roles WHERE rolname = current_user'
+    )
+    skip 'needs a role that can create roles' unless can_create_roles
+
+    connection.execute("DROP ROLE IF EXISTS #{role}")
+    connection.execute("CREATE ROLE #{role}")
+
+    define_table(:rls_entities) do |t|
+      t.string :name
+      t.index :name, unique: true, name: 'index_rls_entities_on_name'
+    end
+    define_table(:rls_scoped_entities) do |t|
+      t.string :name
+      t.string :scope_key
+      t.index %i[scope_key name], unique: true, name: 'index_rls_scoped_entities_on_scope_key_and_name'
+    end
+
+    %w[rls_entities rls_scoped_entities].each do |table|
+      connection.execute("GRANT SELECT, INSERT, UPDATE, DELETE ON #{table} TO #{role}")
+      connection.execute("GRANT USAGE, SELECT ON SEQUENCE #{table}_id_seq TO #{role}")
+      connection.execute("ALTER TABLE #{table} ENABLE ROW LEVEL SECURITY")
+      connection.execute("CREATE POLICY allow_all ON #{table} USING (true) WITH CHECK (true)")
+    end
+    connection.execute("SET ROLE #{role}")
+  end
+
+  after do
+    connection.execute('RESET ROLE')
+    connection.execute("DROP OWNED BY #{role}")
+    connection.execute("DROP ROLE #{role}")
+  end
+
+  it 'is run under a role that row-level security applies to (the premise of the other examples)' do
+    connection.execute("INSERT INTO rls_entities (name) VALUES ('taken')")
+
+    error = begin
+      connection.execute("INSERT INTO rls_entities (name) VALUES ('taken')")
+    rescue ActiveRecord::RecordNotUnique => e
+      e
+    end
+
+    expect(error.message).to include('unique constraint "index_rls_entities_on_name"')
+    expect(error.message).not_to match(/Key \(/)
+  end
+
+  shared_examples 'turns the collision into a validation error' do
+    it 'adds the error to the record instead of raising' do
+      klass.create!(attrs)
+
+      record = klass.create(attrs)
+
+      expect(record.errors.details).to eq(error_details)
+    end
+
+    it 'raises RecordInvalid from the bang methods' do
+      klass.create!(attrs)
+
+      expect { klass.create!(attrs) }.to raise_error(ActiveRecord::RecordInvalid)
+    end
+  end
+
+  context 'when the unique index is found by the attribute' do
+    let(:klass) { define_class(ActiveRecord::Base, :rls_entities) { validates_db_uniqueness_of :name } }
+    let(:attrs) { { name: 'taken' } }
+    let(:error_details) { { name: [{ error: :taken, value: 'taken' }] } }
+
+    include_examples 'turns the collision into a validation error'
+  end
+
+  context 'when the unique index is found by the attribute and its scope' do
+    let(:klass) { define_class(ActiveRecord::Base, :rls_scoped_entities) { validates_db_uniqueness_of :name, scope: :scope_key } }
+    let(:attrs) { { name: 'taken', scope_key: 'one' } }
+    let(:error_details) { { name: [{ error: :taken, value: 'taken' }] } }
+
+    include_examples 'turns the collision into a validation error'
+  end
+
+  context 'when the unique index is named' do
+    let(:klass) do
+      define_class(ActiveRecord::Base, :rls_entities) do
+        validates_db_uniqueness_of :name, index_name: :index_rls_entities_on_name
+      end
+    end
+    let(:attrs) { { name: 'taken' } }
+    let(:error_details) { { name: [{ error: :taken, value: 'taken' }] } }
+
+    include_examples 'turns the collision into a validation error'
+  end
+end