diff --git a/src/Payload.php b/src/Payload.php
index 4994b05..ce4b4b0 100644
--- a/src/Payload.php
+++ b/src/Payload.php
@@ -348,7 +348,7 @@ public function jsonSerialize()
             $payload[self::PAYLOAD_ROOT_KEY][self::PAYLOAD_THREAD_ID_KEY] = $this->threadId;
         }
 
-        if (count($this->customValues)) {
+        if ((is_array($this->customValues) || $this->customValues instanceof Countable) && count($this->customValues)) {
             $payload = array_merge($payload, $this->customValues);
         }