diff --git a/spec/functions/is_string_spec.rb b/spec/functions/is_string_spec.rb
index 1abe5bc..946305f 100644
--- a/spec/functions/is_string_spec.rb
+++ b/spec/functions/is_string_spec.rb
@@ -1,47 +1,47 @@
 require 'spec_helper'
 
 describe 'is_string' do
   it { is_expected.not_to eq(nil) }
   it { is_expected.to run.with_params.and_raise_error(Puppet::ParseError, %r{wrong number of arguments}i) }
   it {
     pending('Current implementation ignores parameters after the first.')
     is_expected.to run.with_params('', '').and_raise_error(Puppet::ParseError, %r{wrong number of arguments}i)
   }
 
   it { is_expected.to run.with_params(3).and_return(false) }
   it { is_expected.to run.with_params('3').and_return(false) }
   it { is_expected.to run.with_params(-3).and_return(false) }
   it { is_expected.to run.with_params('-3').and_return(false) }
 
   it { is_expected.to run.with_params(3.7).and_return(false) }
   it { is_expected.to run.with_params('3.7').and_return(false) }
   it { is_expected.to run.with_params(-3.7).and_return(false) }
   it { is_expected.to run.with_params('-3.7').and_return(false) }
 
-  it { is_expected.to run.with_params(undef).and_return(false) }
+  it { is_expected.to run.with_params(:undef).and_return(false) }
   it { is_expected.to run.with_params([]).and_return(false) }
   it { is_expected.to run.with_params([1]).and_return(false) }
   it { is_expected.to run.with_params({}).and_return(false) }
   it { is_expected.to run.with_params(true).and_return(false) }
   it { is_expected.to run.with_params(false).and_return(false) }
   it { is_expected.to run.with_params('one').and_return(true) }
   it { is_expected.to run.with_params('0001234').and_return(true) }
   it { is_expected.to run.with_params('aaa' => 'www.com').and_return(false) }
 
   context 'with  deprecation warning' do
     after(:each) do
       ENV.delete('STDLIB_LOG_DEPRECATIONS')
     end
     # Checking for deprecation warning, which should only be provoked when the env variable for it is set.
     it 'displays a single deprecation' do
       ENV['STDLIB_LOG_DEPRECATIONS'] = 'true'
       expect(scope).to receive(:warning).with(include('This method is deprecated'))
       is_expected.to run.with_params('sponge').and_return(true)
     end
     it 'displays no warning for deprecation' do
       ENV['STDLIB_LOG_DEPRECATIONS'] = 'false'
       expect(scope).to receive(:warning).with(include('This method is deprecated')).never
       is_expected.to run.with_params('bob').and_return(true)
     end
   end
 end