diff --git a/src/EncodingFactory.php b/src/EncodingFactory.php
index c7ab938..f241eaa 100644
--- a/src/EncodingFactory.php
+++ b/src/EncodingFactory.php
@@ -70,7 +70,7 @@ class EncodingFactory
     public static function registerModelToEncoding(string $modelName, string $encodingName)
     {
         if (array_key_exists($modelName, self::$modelToEncoding)) {
-            throw new Exception("{$modelName} already exists");
+            throw new InvalidArgumentException("\"{$modelName}\" already exists in map");
         }
 
         self::$modelToEncoding[$modelName] = $encodingName;
diff --git a/tests/EncodingFactoryTest.php b/tests/EncodingFactoryTest.php
index 33449f0..66d4a79 100644
--- a/tests/EncodingFactoryTest.php
+++ b/tests/EncodingFactoryTest.php
@@ -46,6 +46,22 @@ public function testCreateByModelNameWithNonExist()
         EncodingFactory::createByModelName('danny');
     }
 
+    public function testRegisterModelToEncoding()
+    {
+        EncodingFactory::registerModelToEncoding('gpt-9000', 'cl100k_base');
+
+        $enc = EncodingFactory::createByModelName('gpt-9000');
+        $this->assertSame('cl100k_base', $enc->getName());
+    }
+
+    public function testRegisterModelToEncodingAlreadyExists()
+    {
+        $this->expectException(InvalidArgumentException::class);
+        $this->expectExceptionMessage('"gpt-4" already exists in map');
+
+        EncodingFactory::registerModelToEncoding('gpt-4', 'cl100k_base');
+    }
+
     public function testRegisterEncoding()
     {
         EncodingFactory::registerEncoding('danny', function () {