diff --git a/app/models/calculator/flexi_rate.rb b/app/models/calculator/flexi_rate.rb
index bb6bb1ff9b..e8bb81458c 100644
--- a/app/models/calculator/flexi_rate.rb
+++ b/app/models/calculator/flexi_rate.rb
@@ -13,6 +13,10 @@ module Calculator
     localize_number :preferred_first_item,
                     :preferred_additional_item
 
+    validates :preferred_first_item,
+              :preferred_additional_item,
+              numericality: { message: :calculator_preferred_value_error }
+
     def self.description
       I18n.t(:flexible_rate)
     end
diff --git a/spec/models/calculator/flexi_rate_spec.rb b/spec/models/calculator/flexi_rate_spec.rb
index 81bbc95ee6..481df1a93e 100644
--- a/spec/models/calculator/flexi_rate_spec.rb
+++ b/spec/models/calculator/flexi_rate_spec.rb
@@ -12,6 +12,16 @@ describe Calculator::FlexiRate do
     )
   end
 
+  it do
+    should validate_numericality_of(:preferred_first_item).
+      with_message("Invalid input. Please use only numbers. For example: 10, 5.5, -20")
+  end
+
+  it do
+    should validate_numericality_of(:preferred_additional_item).
+      with_message("Invalid input. Please use only numbers. For example: 10, 5.5, -20")
+  end
+
   context 'when nb of items ordered is above preferred max' do
     let(:quantity) { 4.0 }