diff --git a/src/Symfony/Component/Mailer/Transport/Smtp/Stream/AbstractStream.php b/src/Symfony/Component/Mailer/Transport/Smtp/Stream/AbstractStream.php
index 55a2594ac1de5..0d3de1400af3e 100644
--- a/src/Symfony/Component/Mailer/Transport/Smtp/Stream/AbstractStream.php
+++ b/src/Symfony/Component/Mailer/Transport/Smtp/Stream/AbstractStream.php
@@ -87,7 +87,7 @@ public function readLine(): string
                 throw new TransportException(sprintf('Connection to "%s" has been closed unexpectedly.', $this->getReadConnectionDescription()));
             }
             if (false === $line) {
-                throw new TransportException(sprintf('Unable to read from connection to "%s": ', $this->getReadConnectionDescription()).error_get_last()['message']);
+                throw new TransportException(sprintf('Unable to read from connection to "%s": ', $this->getReadConnectionDescription().error_get_last()['message'] ?? ''));
             }
         }