diff --git a/lib/girocode/code.rb b/lib/girocode/code.rb
index b2aae82..c42cdfb 100644
--- a/lib/girocode/code.rb
+++ b/lib/girocode/code.rb
@@ -59,7 +59,7 @@ def amount=(value)
 
     def purpose=(value)
       unless value.nil?
-        raise ArgumentError, "invalid purpose #{value.inspect}" unless value.match?(/\A[A-z0-9]{0,4}\z/)
+        raise ArgumentError, "invalid purpose #{value.inspect}" unless value.match?(/\A[A-Za-z0-9]{0,4}\z/)
       end
       @purpose = value
     end
diff --git a/test/girocode_test.rb b/test/girocode_test.rb
index 166c990..9967e8e 100644
--- a/test/girocode_test.rb
+++ b/test/girocode_test.rb
@@ -30,4 +30,10 @@ def test_only_one_reference
     end
   end
 
+  def test_purpose_rejects_gap_characters
+    assert_raises ArgumentError do
+      Girocode.new(bic: 'BHBLDEHHXXX', name: 'Franz Mustermann', iban: 'DE71110220330123456789', currency: :eur, amount: 12.3, purpose: '[')
+    end
+  end
+
 end