diff --git a/dom/webidl/MozVoicemailStatus.webidl b/dom/webidl/MozVoicemailStatus.webidl
index c105328c6e1..7e008b96f43 100644
--- a/dom/webidl/MozVoicemailStatus.webidl
+++ b/dom/webidl/MozVoicemailStatus.webidl
@@ -37,11 +37,11 @@ interface MozVoicemailStatus
    * Return call number received for this voicemail status, or null if one
    * wasn't provided.
    */
-  readonly attribute DOMString returnNumber;
+  readonly attribute DOMString? returnNumber;
 
   /**
    * Displayable return call message received for this voicemail status, or null
    * if one wasn't provided.
    */
-  readonly attribute DOMString returnMessage;
+  readonly attribute DOMString? returnMessage;
 };