diff --git a/tests/Support/SupportStrTest.php b/tests/Support/SupportStrTest.php
index 74469575bc05..4183d608ef78 100755
--- a/tests/Support/SupportStrTest.php
+++ b/tests/Support/SupportStrTest.php
@@ -302,6 +302,29 @@ public function testStrContainsAll($haystack, $needles, $expected, $ignoreCase =
         $this->assertEquals($expected, Str::containsAll($haystack, $needles, $ignoreCase));
     }
 
+    public function testConvertCase()
+    {
+        // Test Upper Case Conversion
+        $this->assertSame('HELLO', Str::convertCase('hello', MB_CASE_UPPER));
+        $this->assertSame('WORLD', Str::convertCase('WORLD', MB_CASE_UPPER));
+
+        // Test Lower Case Conversion
+        $this->assertSame('hello', Str::convertCase('HELLO', MB_CASE_LOWER));
+        $this->assertSame('world', Str::convertCase('WORLD', MB_CASE_LOWER));
+
+        // Test Case Folding
+        $this->assertSame('hello', Str::convertCase('HeLLo', MB_CASE_FOLD));
+        $this->assertSame('world', Str::convertCase('WoRLD', MB_CASE_FOLD));
+
+        // Test Multi-byte String
+        $this->assertSame('ÜÖÄ', Str::convertCase('üöä', MB_CASE_UPPER, 'UTF-8'));
+        $this->assertSame('üöä', Str::convertCase('ÜÖÄ', MB_CASE_LOWER, 'UTF-8'));
+
+        // Test Unsupported Mode (Let's assume '-1' is unsupported)
+        $this->assertSame('Hello', Str::convertCase('Hello', -1));
+    }
+
+
     public function testParseCallback()
     {
         $this->assertEquals(['Class', 'method'], Str::parseCallback('Class@method', 'foo'));