diff --git a/src/GW2Api.php b/src/GW2Api.php
index e6a5737..67dd53b 100644
--- a/src/GW2Api.php
+++ b/src/GW2Api.php
@@ -51,7 +51,18 @@ protected function getOptions( array $options = [] ) {
         ] + $options;
     }
 
+    /**
+     * @param string $handler
+     */
     public function registerHandler( $handler ) {
+        if( is_null( $handler )) {
+            throw new \InvalidArgumentException( '$handler can\'t be null' );
+        }
+
+        if( !is_string( $handler )) {
+            throw new \InvalidArgumentException( '$handler has to be a string (class name of a valid ApiHandler)' );
+        }
+
         $handlerClass = new \ReflectionClass( $handler );
         if( !$handlerClass->isSubclassOf( '\GW2Treasures\GW2Api\V2\ApiHandler' )) {
             throw new \InvalidArgumentException( '$handler has to be a ApiHandler');
diff --git a/tests/ApiHandlerTest.php b/tests/ApiHandlerTest.php
index 066ce18..46e2528 100644
--- a/tests/ApiHandlerTest.php
+++ b/tests/ApiHandlerTest.php
@@ -39,6 +39,28 @@ public function testAsJson() {
         $invalidNoContentType = $this->makeResponse( '{"valid":false}', null );
         $this->assertNull( $handler->responseAsJson( $invalidNoContentType ));
     }
+
+    /**
+     * @expectedException \InvalidArgumentException
+     */
+    public function testRegisterNull() {
+        $this->api()->registerHandler( null );
+    }
+
+
+    /**
+     * @expectedException \InvalidArgumentException
+     */
+    public function testRegisterSubclassOfHandler() {
+        $this->api()->registerHandler( new stdClass() );
+    }
+
+    /**
+     * @expectedException \InvalidArgumentException
+     */
+    public function testRegisterHandler() {
+        $this->api()->registerHandler( $this->getHandler( $this->getEndpoint() ) );
+    }
 }
 
 class TestHandler extends ApiHandler {