diff --git a/src/CreditCard.php b/src/CreditCard.php
index 242f63802..75d659456 100644
--- a/src/CreditCard.php
+++ b/src/CreditCard.php
@@ -279,7 +279,7 @@ public function isValid($value)
         $foundl = false;
         foreach ($types as $type) {
             foreach ($this->cardType[$type] as $prefix) {
-                if (substr($value, 0, strlen($prefix)) == $prefix) {
+                if (0 === strpos($value, $prefix)) {
                     $foundp = true;
                     if (in_array($length, $this->cardLength[$type])) {
                         $foundl = true;