diff --git a/.phpstan.neon b/.phpstan.neon
index fca7f6d59d..2a89c7ae1a 100644
--- a/.phpstan.neon
+++ b/.phpstan.neon
@@ -6,13 +6,14 @@ parameters:
     level: 2
 
     paths:
-        # - addon/
+        - addon/
         - src/
 
     excludePaths:
         analyse:
             - addon/*/lang/*
             - addon/*/vendor/*
+            - addon/convert/UnitConvertor.php
             - addon/pumpio/oauth/*
 
     scanDirectories:
@@ -39,3 +40,27 @@ parameters:
             # Ignore missing IMAP\Connection class in PHP <= 8.0
             message: '(^Parameter .+ has invalid type IMAP\\Connection\.$)'
             path: src
+
+        -
+            # Ignore missing SMTP class in PHPMailer 5.2.21
+            # see https://github.com/PHPMailer/PHPMailer/blob/v5.2.21/class.smtp.php
+            message: '(^.+ an unknown class SMTP\.$)'
+            path: addon/mailstream/phpmailer
+
+        -
+            # Ignore missing SMTP class in PHPMailer 5.2.21
+            # see https://github.com/PHPMailer/PHPMailer/blob/v5.2.21/class.smtp.php
+            message: '(^Property .+ has unknown class SMTP as its type\.$)'
+            path: addon/mailstream/phpmailer
+
+        -
+            # Ignore missing SMTP class in PHPMailer 5.2.21
+            # see https://github.com/PHPMailer/PHPMailer/blob/v5.2.21/class.smtp.php
+            message: '(^Method .+ has invalid return type SMTP\.$)'
+            path: addon/mailstream/phpmailer
+
+        -
+            # Ignore missing SMTP class in PHPMailer 5.2.21
+            # see https://github.com/PHPMailer/PHPMailer/blob/v5.2.21/class.smtp.php
+            message: '(^Instantiated class SMTP not found\.$)'
+            path: addon/mailstream/phpmailer
diff --git a/src/Util/Strings.php b/src/Util/Strings.php
index 46d10529a9..fbf691879e 100644
--- a/src/Util/Strings.php
+++ b/src/Util/Strings.php
@@ -530,8 +530,12 @@ class Strings
 	{
 		$shorthand = trim($shorthand);
 
-		if (is_numeric($shorthand)) {
-			return $shorthand;
+		if (ctype_digit($shorthand)) {
+			return (int) $shorthand;
+		}
+
+		if ($shorthand === '') {
+			return 0;
 		}
 
 		$last      = strtolower($shorthand[strlen($shorthand) - 1]);