diff --git a/tests/Mouson/Helpers/SequenceCalculator/DoubleSequenceCalculatorTest.php b/tests/Mouson/Helpers/SequenceCalculator/DoubleSequenceCalculatorTest.php
index 2bdd15e..a9de5e2 100644
--- a/tests/Mouson/Helpers/SequenceCalculator/DoubleSequenceCalculatorTest.php
+++ b/tests/Mouson/Helpers/SequenceCalculator/DoubleSequenceCalculatorTest.php
@@ -32,4 +32,19 @@ public function test_測試雙倍數列輸入0應回傳0()
         /** Assert */
         $this->assertEquals($expected, $actual);
     }
+
+    public function test_測試雙倍數列輸入2應回傳4()
+    {
+        /** Arrange */
+        $target = new DoubleSequenceCalculator();
+        $value  = 2;
+        /** Assume */
+        $expected = 4;
+
+        /** Act */
+        $actual = $target->calculate($value);
+
+        /** Assert */
+        $this->assertEquals($expected, $actual);
+    }
 }