diff --git a/src/Json/JsonProtocol.php b/src/Json/JsonProtocol.php
index fafa32b..b865ae9 100644
--- a/src/Json/JsonProtocol.php
+++ b/src/Json/JsonProtocol.php
@@ -91,6 +91,9 @@ class JsonProtocol implements ProtocolInterface
     {
         // check for $this->prependSizeBytes
         if ($this->prependSizeBytes > 0) {
+            if (strlen($data) < $this->prependSizeBytes) {
+                return null;
+            }
             $len = $this->unpackSizeBytes($data);
             // if size is greater than data (i.e. incomplete)
             if ($len > strlen($data) - $this->prependSizeBytes) return null;