diff --git a/lib/datatrans/json/transaction/merchant_authorize.rb b/lib/datatrans/json/transaction/merchant_authorize.rb
index 8488325..c0fcd45 100644
--- a/lib/datatrans/json/transaction/merchant_authorize.rb
+++ b/lib/datatrans/json/transaction/merchant_authorize.rb
@@ -27,13 +27,17 @@ def process
     def request_body
       auto_settle = params[:auto_settle].nil? ? true : params[:auto_settle]
 
-      {
+      body = {
         currency: params[:currency],
         refno: params[:refno],
         amount: params[:amount],
-        card: params[:card],
         autoSettle: auto_settle
       }
+
+      body[:card] = params[:card] if params[:card].present?
+      body[:TWI] = params[:TWI] if params[:TWI].present?
+
+      body
     end
   end
 
diff --git a/spec/json/merchant_authorize_spec.rb b/spec/json/merchant_authorize_spec.rb
index ad04cc9..fbdc722 100644
--- a/spec/json/merchant_authorize_spec.rb
+++ b/spec/json/merchant_authorize_spec.rb
@@ -29,6 +29,16 @@
       auto_settle: true
     }
 
+    @valid_params_twint = {
+      currency: "CHF",
+      refno: "B4B4B4B4B",
+      amount: 1000,
+      TWI: {
+        alias: "123456"
+      },
+      auto_settle: true
+    }
+
     @expected_request_body = {
       currency: "CHF",
       refno: "B4B4B4B4B",
@@ -41,6 +51,16 @@
       autoSettle: true
     }
 
+    @expected_request_body_twint = {
+      currency: "CHF",
+      refno: "B4B4B4B4B",
+      amount: 1000,
+      TWI: {
+        alias: "123456"
+      },
+      autoSettle: true
+    }
+
     @invalid_params = {
       currency: "CHF",
       refno: "B4B4B4B4B",
@@ -78,6 +98,14 @@
     end
   end
 
+  context "with TWI specified" do
+    it "handles TWI correctly in request_body" do
+      request = Datatrans::JSON::Transaction::MerchantAuthorize.new(@datatrans, @valid_params_twint)
+
+      expect(request.request_body).to eq(@expected_request_body_twint)
+    end
+  end
+
   context "failed response" do
     before do
       allow_any_instance_of(Datatrans::JSON::Transaction::MerchantAuthorize).to receive(:process).and_return(@failed_response)