diff --git a/src/Numbers/Number.php b/src/Numbers/Number.php
index e6abf6e0..ddce5ba7 100644
--- a/src/Numbers/Number.php
+++ b/src/Numbers/Number.php
@@ -296,6 +296,6 @@ public function setAppId(string $appId): self
     public function getAppId(): ?string
     {
         // These should never be different, but might not both be set
-        return $this->data['voiceCallbackValue'] ?? $this->data['messagesCallbackValue'];
+        return $this->data['voiceCallbackValue'] ?? $this->data['messagesCallbackValue'] ?? null;
     }
 }