diff --git a/src/Spryng/SpryngRestApi/Utils.php b/src/Spryng/SpryngRestApi/Utils.php
index 6d6cd7d..be1ce29 100644
--- a/src/Spryng/SpryngRestApi/Utils.php
+++ b/src/Spryng/SpryngRestApi/Utils.php
@@ -17,10 +17,10 @@ class Utils
     public static function assert($expected, $actual = null)
     {
         if ($actual === null && !$expected) {
-            throw new ValidationException('\'%s\' cannot be null', $expected);
+            throw new ValidationException(sprintf('\'%s\' cannot be null', $expected));
         }
         if (!$expected || $actual !== null && $expected !== $actual) {
             throw new ValidationException(sprintf('Assertion failed. Got \'%s\' but expected \'%s\'.', $actual, $expected));
         }
     }
-}
\ No newline at end of file
+}