diff --git a/phpseclib/Crypt/Common/SymmetricKey.php b/phpseclib/Crypt/Common/SymmetricKey.php
index 31bb2e87..00bfdd45 100644
--- a/phpseclib/Crypt/Common/SymmetricKey.php
+++ b/phpseclib/Crypt/Common/SymmetricKey.php
@@ -668,11 +668,11 @@ abstract class SymmetricKey
             switch (true) {
                 // PHP_OS & "\xDF\xDF\xDF" == strtoupper(substr(PHP_OS, 0, 3)), but a lot faster
                 case (PHP_OS & "\xDF\xDF\xDF") === 'WIN':
-                case (php_uname('m') & "\xDF\xDF\xDF") != 'ARM':
+                case !(is_string(php_uname('m')) && (php_uname('m') & "\xDF\xDF\xDF") == 'ARM'):
                 case defined('PHP_INT_SIZE') && PHP_INT_SIZE == 8:
                     self::$use_reg_intval = true;
                     break;
-                case (php_uname('m') & "\xDF\xDF\xDF") == 'ARM':
+                case is_string(php_uname('m')) && (php_uname('m') & "\xDF\xDF\xDF") == 'ARM':
                     switch (true) {
                         /* PHP 7.0.0 introduced a bug that affected 32-bit ARM processors: