diff --git a/htdocs/core/class/smtps.class.php b/htdocs/core/class/smtps.class.php
index 135d9d9e8de..46193e1efea 100644
--- a/htdocs/core/class/smtps.class.php
+++ b/htdocs/core/class/smtps.class.php
@@ -1862,6 +1862,7 @@ class SMTPs
 		} elseif ($type == 'alternative') {
 			return $this->_smtpsAlternativeBoundary;
 		}
+		return '';
 	}
 
 	// phpcs:disable PEAR.NamingConventions.ValidFunctionName.ScopeNotCamelCaps