diff --git a/Mpdf/SizeConverter.php b/Mpdf/SizeConverter.php
index 9ec93fd..217d4aa 100755
--- a/Mpdf/SizeConverter.php
+++ b/Mpdf/SizeConverter.php
@@ -30,7 +30,7 @@ class SizeConverter implements \Psr\Log\LoggerAwareInterface
 		$this->logger = $logger;
 	}
 
-	public function setLogger(LoggerInterface $logger)
+	public function setLogger(LoggerInterface $logger) : void
 	{
 		$this->logger = $logger;
 	}