diff --git a/src/Packer/JsonPacker.php b/src/Packer/JsonPacker.php
index 1e1eac4..1fa58e8 100644
--- a/src/Packer/JsonPacker.php
+++ b/src/Packer/JsonPacker.php
@@ -54,7 +54,7 @@ public function pack($value)
      */
     public function unpack($packed)
     {
-        if (is_string($packed)) {
+        if (!is_string($packed)) {
             throw new InvalidArgumentException("packed value should be a string");
         }