diff --git a/src/Mpdf.php b/src/Mpdf.php
index e17d43005..ff56d450e 100644
--- a/src/Mpdf.php
+++ b/src/Mpdf.php
@@ -610,7 +610,7 @@ class Mpdf implements \Psr\Log\LoggerAwareInterface
 
 	var $directionality;
 
-	var $extgstates; // Used for alpha channel - Transparency (Watermark)
+	var $extgstates = []; // Used for alpha channel - Transparency (Watermark)
 	var $mgl;
 	var $mgt;
 	var $mgr;
diff --git a/tests/Mpdf/ConfigurationTest.php b/tests/Mpdf/ConfigurationTest.php
index 78fb79d6e..fed02dd66 100644
--- a/tests/Mpdf/ConfigurationTest.php
+++ b/tests/Mpdf/ConfigurationTest.php
@@ -14,6 +14,17 @@ public function testDefaultSettings()
 		$this->assertFalse($mpdf->autoPadding);
 	}
 
+	/**
+	 * The writers count() this before anything has drawn, so an instance that never ran the
+	 * constructor - a partial mock, say - must still find an array here
+	 */
+	public function testExtgstatesDefaultsToAnArray()
+	{
+		$mpdf = (new \ReflectionClass(Mpdf::class))->newInstanceWithoutConstructor();
+
+		$this->assertSame([], $mpdf->extgstates);
+	}
+
 	public function testOverwrittenSettings()
 	{
 		$mpdf = new Mpdf([