diff --git a/lib/Assert/Assertion.php b/lib/Assert/Assertion.php
index 243e64d..c3dcc2c 100644
--- a/lib/Assert/Assertion.php
+++ b/lib/Assert/Assertion.php
@@ -1857,7 +1857,7 @@ public static function false($value, $message = null, string $propertyPath = nul
      */
     public static function classExists($value, $message = null, string $propertyPath = null): bool
     {
-        if (!\class_exists($value)) {
+        if (!\is_string($value) || !\class_exists($value)) {
             $message = \sprintf(
                 static::generateMessage($message ?: 'Class "%s" does not exist.'),
                 static::stringify($value)