diff --git a/src/Config.php b/src/Config.php
index 8a94edf22c..cfbae896b9 100644
--- a/src/Config.php
+++ b/src/Config.php
@@ -198,6 +198,23 @@ class Config
      */
     private static $executablePaths = [];
 
+    /**
+     * A list of valid generators.
+     *
+     * - Keys: lowercase version of the generator name.
+     * - Values: name of the generator PHP class.
+     *
+     * Note: once support for PHP < 5.6 is dropped, this property should be refactored into a class
+     * constant.
+     *
+     * @var array<string, string>
+     */
+    private static $validGenerators = [
+        'text'     => 'Text',
+        'html'     => 'HTML',
+        'markdown' => 'Markdown',
+    ];
+
 
     /**
      * Get the value of an inaccessible property.
@@ -1233,20 +1250,21 @@ public function processLongArgument($arg, $pos)
                     break;
                 }
 
-                $generatorName   = substr($arg, 10);
-                $validGenerators = [
-                    'Text',
-                    'HTML',
-                    'Markdown',
-                ];
-
-                if (in_array($generatorName, $validGenerators, true) === false) {
-                    $error  = 'ERROR: "'.$generatorName.'" is not a valid generator. Valid options are: Text, HTML, and Markdown.'.PHP_EOL.PHP_EOL;
-                    $error .= $this->printShortUsage(true);
+                $generatorName          = substr($arg, 10);
+                $lowerCaseGeneratorName = strtolower($generatorName);
+
+                if (isset(self::$validGenerators[$lowerCaseGeneratorName]) === false) {
+                    $validOptions = implode(', ', array_values(self::$validGenerators));
+                    $error        = sprintf(
+                        'ERROR: "%s" is not a valid generator. Valid options are: %s.'.PHP_EOL.PHP_EOL,
+                        $generatorName,
+                        $validOptions
+                    );
+                    $error       .= $this->printShortUsage(true);
                     throw new DeepExitException($error, 3);
                 }
 
-                $this->generator = $generatorName;
+                $this->generator = self::$validGenerators[$lowerCaseGeneratorName];
                 self::$overriddenDefaults['generator'] = true;
             } else if (substr($arg, 0, 9) === 'encoding=') {
                 if (isset(self::$overriddenDefaults['encoding']) === true) {
diff --git a/tests/Core/Config/GeneratorArgTest.php b/tests/Core/Config/GeneratorArgTest.php
index 389d3e4af4..72949409e9 100644
--- a/tests/Core/Config/GeneratorArgTest.php
+++ b/tests/Core/Config/GeneratorArgTest.php
@@ -23,17 +23,18 @@ final class GeneratorArgTest extends TestCase
     /**
      * Ensure that the generator property is set when the parameter is passed a valid value.
      *
-     * @param string $generatorName Generator name.
+     * @param string $argumentValue         Generator name passed in the command line.
+     * @param string $expectedPropertyValue Expected value of the generator property.
      *
      * @dataProvider dataValidGeneratorNames
      *
      * @return void
      */
-    public function testValidGenerators($generatorName)
+    public function testValidGenerators($argumentValue, $expectedPropertyValue)
     {
-        $config = new ConfigDouble(["--generator=$generatorName"]);
+        $config = new ConfigDouble(["--generator=$argumentValue"]);
 
-        $this->assertSame($generatorName, $config->generator);
+        $this->assertSame($expectedPropertyValue, $config->generator);
 
     }//end testValidGenerators()
 
@@ -48,9 +49,30 @@ public function testValidGenerators($generatorName)
     public static function dataValidGeneratorNames()
     {
         return [
-            ['Text'],
-            ['HTML'],
-            ['Markdown'],
+            [
+                'Text',
+                'Text',
+            ],
+            [
+                'HTML',
+                'HTML',
+            ],
+            [
+                'Markdown',
+                'Markdown',
+            ],
+            [
+                'TEXT',
+                'Text',
+            ],
+            [
+                'tEXt',
+                'Text',
+            ],
+            [
+                'html',
+                'HTML',
+            ],
         ];
 
     }//end dataValidGeneratorNames()
@@ -88,7 +110,7 @@ public function testOnlySetOnce()
     public function testInvalidGenerator($generatorName)
     {
         $exception = 'PHP_CodeSniffer\Exceptions\DeepExitException';
-        $message   = 'ERROR: "'.$generatorName.'" is not a valid generator. Valid options are: Text, HTML, and Markdown.';
+        $message   = 'ERROR: "'.$generatorName.'" is not a valid generator. Valid options are: Text, HTML, Markdown.';
 
         if (method_exists($this, 'expectException') === true) {
             // PHPUnit 5+.
@@ -116,7 +138,6 @@ public static function dataInvalidGeneratorNames()
         return [
             ['InvalidGenerator'],
             ['Text,HTML'],
-            ['TEXT'],
             [''],
         ];