diff --git a/lib/spanish_vat_validators.rb b/lib/spanish_vat_validators.rb
index c5c9bcd..d311478 100644
--- a/lib/spanish_vat_validators.rb
+++ b/lib/spanish_vat_validators.rb
@@ -1,3 +1,4 @@
+#encoding: utf-8
 require "spanish_vat_validators/version"
 
 module ActiveModel::Validations
@@ -8,16 +9,20 @@ def message(kind='vat')
     end
 
     # Validates NIF
-    def validate_nif(value)
+    def validate_nif(v)
+      return false if v.nil? || v.empty?
+      value = v.upcase
       return false unless value.match(/[0-9]{8}[a-z]/i)
       letters = "TRWAGMYFPDXBNJZSQVHLCKE"
-      check = value.slice!(value.length - 1..value.length - 1).upcase
+      check = value.slice!(value.length - 1..value.length - 1)
       calculated_letter = letters[value.to_i % 23].chr
       return check === calculated_letter
     end
 
     # Validates CIF
-    def validate_cif(value)
+    def validate_cif(v)
+      return false if v.nil? || v.empty?
+      value = v.clone
       return false unless value.match(/[a-wyz][0-9]{7}[0-9a-z]/i)
       pares = 0
       impares = 0
@@ -49,7 +54,9 @@ def validate_cif(value)
 
     # Validates NIE
     # a NIE is really a NIF with first number changed to capital X, Y or Z letter
-    def validate_nie(value)
+    def validate_nie(v)
+	  return false if v.nil? || v.empty?
+      value = v.upcase
       return false unless value.match(/[xyz][0-9]{7}[a-z]/i)
       value[0] = {"X" => "0", "Y" => "1", "Z" => "2"}[value[0]]
       validate_nif(value)
@@ -60,7 +67,7 @@ def validate_nie(value)
   class ValidSpanishVatValidator < ActiveModel::EachValidator
     include SpanishVatValidatorsHelpers
     def validate_each(record, attribute, value)
-      record.errors[attribute] = message unless validate_nif(value.clone) or validate_cif(value.clone) or validate_nie(value.clone)
+      record.errors[attribute] = message unless validate_nif(value) or validate_cif(value) or validate_nie(value)
     end
   end
 
@@ -68,7 +75,7 @@ def validate_each(record, attribute, value)
   class ValidSpanishIdValidator < ActiveModel::EachValidator
     include SpanishVatValidatorsHelpers
     def validate_each(record, attribute, value)
-      record.errors[attribute] = message unless validate_nif(value.clone) or validate_nie(value.clone)
+      record.errors[attribute] = message unless validate_nif(value) or validate_nie(value)
     end
   end
 
@@ -76,7 +83,7 @@ def validate_each(record, attribute, value)
   class ValidNifValidator < ActiveModel::EachValidator
     include SpanishVatValidatorsHelpers
     def validate_each(record, attribute,value)
-      record.errors[attribute] = message('nif') unless validate_nif(value.clone)
+      record.errors[attribute] = message('nif') unless validate_nif(value)
     end
   end
 
@@ -84,7 +91,7 @@ def validate_each(record, attribute,value)
   class ValidCifValidator < ActiveModel::EachValidator
     include SpanishVatValidatorsHelpers
     def validate_each(record, attribute,value)
-      record.errors[attribute] = message('cif') unless validate_cif(value.clone)
+      record.errors[attribute] = message('cif') unless validate_cif(value)
     end
   end
 
@@ -92,7 +99,7 @@ def validate_each(record, attribute,value)
   class ValidNieValidator < ActiveModel::EachValidator
     include SpanishVatValidatorsHelpers
     def validate_each(record, attribute,value)
-      record.errors[attribute] = message('nie') unless validate_nie(value.clone)
+      record.errors[attribute] = message('nie') unless validate_nie(value)
     end
   end