diff --git a/tariff/octopus.go b/tariff/octopus.go
index 1a496573a..eb69484a7 100644
--- a/tariff/octopus.go
+++ b/tariff/octopus.go
@@ -48,7 +48,7 @@ func NewOctopusFromConfig(other map[string]interface{}) (api.Tariff, error) {
 		if cc.Region == "" {
 			return nil, errors.New("missing region")
 		}
-		if cc.Tariff == "" {
+		if cc.Tariff != "" {
 			// deprecated - copy to correct slot and WARN
 			logger.WARN.Print("'tariff' is deprecated and will break in a future version - use 'productCode' instead")
 			cc.ProductCode = cc.Tariff
diff --git a/tariff/octopus_test.go b/tariff/octopus_test.go
new file mode 100644
index 000000000..30a4f5245
--- /dev/null
+++ b/tariff/octopus_test.go
@@ -0,0 +1,34 @@
+package tariff
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/require"
+)
+
+func TestOctopusConfigParse(t *testing.T) {
+	// This test will start failing if you remove the deprecated "tariff" config var.
+	validTariffConfig := map[string]interface{}{
+		"region": "H",
+		"tariff": "GO-22-03-29",
+	}
+
+	_, err := NewOctopusFromConfig(validTariffConfig)
+	require.NoError(t, err)
+
+	validProductCodeConfig := map[string]interface{}{
+		"region":      "H",
+		"productcode": "GO-22-03-29",
+	}
+
+	_, err = NewOctopusFromConfig(validProductCodeConfig)
+	require.NoError(t, err)
+
+	invalidApiAndProductCodeConfig := map[string]interface{}{
+		"region":      "H",
+		"productcode": "GO-22-03-29",
+		"apikey":      "nope",
+	}
+	_, err = NewOctopusFromConfig(invalidApiAndProductCodeConfig)
+	require.Error(t, err)
+}