diff --git a/lib/pseudonymisation.rb b/lib/pseudonymisation.rb
index 846e131b8..b4a02dbd5 100644
--- a/lib/pseudonymisation.rb
+++ b/lib/pseudonymisation.rb
@@ -4,7 +4,7 @@ class Pseudonymisation
     return if source.blank?
 
     source.transform_values do |value|
-      of_value(value)
+      of_value(value.to_s)
     end
   end
 
@@ -33,7 +33,7 @@ class Pseudonymisation
   end
 
   def self.of_string(source)
-    return '*' if source.length == 1
+    return '*' if source.to_s.length <= 1
     return "#{source.first}*#{source.last}" if source.exclude?(' ')
 
     source.split.map do |sub_string|
diff --git a/spec/lib/pseudonymisation_spec.rb b/spec/lib/pseudonymisation_spec.rb
index 183ed360e..672e0e4b4 100644
--- a/spec/lib/pseudonymisation_spec.rb
+++ b/spec/lib/pseudonymisation_spec.rb
@@ -7,7 +7,7 @@ RSpec.describe Pseudonymisation do
     let(:source) do
       {
         firstname:    'John',
-        lastname:     'Doe',
+        lastname:     nil,
         email:        'john.doe@example.com',
         organization: 'Example Inc.',
       }
@@ -16,7 +16,7 @@ RSpec.describe Pseudonymisation do
     let(:result) do
       {
         firstname:    'J*n',
-        lastname:     'D*e',
+        lastname:     '*',
         email:        'j*e@e*e.com',
         organization: 'E*e I*.',
       }
@@ -44,6 +44,14 @@ RSpec.describe Pseudonymisation do
         expect(described_class.of_value(source)).to eq('Z*d')
       end
     end
+
+    context 'when nil is given' do
+      let(:source) { nil }
+
+      it 'returns *' do
+        expect(described_class.of_value(source)).to eq('*')
+      end
+    end
   end
 
   describe '.of_email_address' do
@@ -107,5 +115,13 @@ RSpec.describe Pseudonymisation do
         expect(described_class.of_string(source)).to eq('Z*d F*n')
       end
     end
+
+    context 'when nil are given' do
+      let(:source) { nil }
+
+      it 'returns *' do
+        expect(described_class.of_string(source)).to eq('*')
+      end
+    end
   end
 end