diff --git a/core/app/models/spree/calculator.rb b/core/app/models/spree/calculator.rb
index 7c931ec39c1..57a8449c718 100644
--- a/core/app/models/spree/calculator.rb
+++ b/core/app/models/spree/calculator.rb
@@ -21,7 +21,7 @@ class Calculator < Spree::Base
     #                            Spree::ShippingRate)
     # @param ... [args, kwargs] Additional arguments passed to the specific compute method
     #
-    # @return [BigDecimal, Numeric] The computed amount
+    # @return [BigDecimal] The computed amount
     #
     # @raise [NotImplementedError] If the calculator doesn't implement the required compute method
     #
diff --git a/core/app/models/spree/calculator/flat_rate.rb b/core/app/models/spree/calculator/flat_rate.rb
index 20b5b501468..87224d5ab42 100644
--- a/core/app/models/spree/calculator/flat_rate.rb
+++ b/core/app/models/spree/calculator/flat_rate.rb
@@ -11,7 +11,7 @@ def compute(object = nil)
       if object && preferred_currency.casecmp(object.currency).zero?
         preferred_amount
       else
-        0
+        Spree::ZERO
       end
     end
   end
diff --git a/core/spec/models/spree/calculator/flat_rate_spec.rb b/core/spec/models/spree/calculator/flat_rate_spec.rb
index f4955f0ec3c..89b8dbdccf9 100644
--- a/core/spec/models/spree/calculator/flat_rate_spec.rb
+++ b/core/spec/models/spree/calculator/flat_rate_spec.rb
@@ -26,14 +26,18 @@
       calculator.preferred_amount = 100.0
       calculator.preferred_currency = "GBP"
       allow(order).to receive_messages currency: "USD"
-      expect(calculator.compute(order).round(2)).to eq(0.0)
+      result = calculator.compute(order)
+      expect(result).to be_a(BigDecimal)
+      expect(result.round(2)).to eq(0.0)
     end
 
     it "should compute the amount as 0 when currency is blank" do
       calculator.preferred_amount = 100.0
       calculator.preferred_currency = ""
       allow(order).to receive_messages currency: "GBP"
-      expect(calculator.compute(order).round(2)).to eq(0.0)
+      result = calculator.compute(order)
+      expect(result).to be_a(BigDecimal)
+      expect(result.round(2)).to eq(0.0)
     end
 
     it "should compute the amount as the rate when the currencies use different casing" do
@@ -46,7 +50,9 @@
     it "should compute the amount as 0 when there is no object" do
       calculator.preferred_amount = 100.0
       calculator.preferred_currency = "GBP"
-      expect(calculator.compute.round(2)).to eq(0.0)
+      result = calculator.compute
+      expect(result).to be_a(BigDecimal)
+      expect(result.round(2)).to eq(0.0)
     end
   end
 end