diff --git a/core/app/models/spree/calculator/flat_rate.rb b/core/app/models/spree/calculator/flat_rate.rb
index 20b5b501468..e9ded29f05c 100644
--- a/core/app/models/spree/calculator/flat_rate.rb
+++ b/core/app/models/spree/calculator/flat_rate.rb
@@ -9,9 +9,9 @@ class Calculator::FlatRate < Calculator
 
     def compute(object = nil)
       if object && preferred_currency.casecmp(object.currency).zero?
-        preferred_amount
+        BigDecimal(preferred_amount.to_s)
       else
-        0
+        BigDecimal("0")
       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..9828526b0b8 100644
--- a/core/spec/models/spree/calculator/flat_rate_spec.rb
+++ b/core/spec/models/spree/calculator/flat_rate_spec.rb
@@ -48,5 +48,32 @@
       calculator.preferred_currency = "GBP"
       expect(calculator.compute.round(2)).to eq(0.0)
     end
+    context "return type" do
+    it "always returns a BigDecimal when currency matches" do
+      calculator.preferred_amount = 25.0
+      calculator.preferred_currency = "USD"
+      allow(order).to receive_messages currency: "USD"
+      expect(calculator.compute(order)).to be_a(BigDecimal)
+    end
+
+    it "always returns a BigDecimal when currency does not match" do
+      calculator.preferred_amount = 100.0
+      calculator.preferred_currency = "GBP"
+      allow(order).to receive_messages currency: "USD"
+      expect(calculator.compute(order)).to be_a(BigDecimal)
+    end
+
+    it "always returns a BigDecimal when there is no object" do
+      calculator.preferred_amount = 100.0
+      expect(calculator.compute).to be_a(BigDecimal)
+    end
+
+    it "always returns a BigDecimal when amount is zero" do
+      calculator.preferred_amount = 0
+      calculator.preferred_currency = "USD"
+      allow(order).to receive_messages currency: "USD"
+      expect(calculator.compute(order)).to be_a(BigDecimal)
+    end
+  end
   end
 end