--- a/calendar/providers/caldav/calDavRequestHandlers.js
+++ b/calendar/providers/caldav/calDavRequestHandlers.js
@@ -586,17 +586,18 @@ webDavSyncHandler.prototype = {
                     this.itemsReported[r.href] = r.getetag;
                     let itemId = this.calendar.mHrefIndex[r.href];
                     let oldEtag = (itemId && this.calendar.mItemInfoCache[itemId].etag);
 
                     if (!oldEtag || oldEtag != r.getetag) {
                         // Etag mismatch, getting new/updated item.
                         this.itemsNeedFetching.push(r.href);
                     }
-                } else if (r.status.indexOf(" 507") > -1) {
+                } else if (r.status &&
+                            r.status.indexOf(" 507") > -1) {
                     // webdav-sync says that if a 507 is encountered and the
                     // url matches the request, the current token should be
                     // saved and another request should be made. We don't
                     // actually compare the URL, its too easy to get this
                     // wrong.
 
                     // The 507 doesn't mean the data received is invalid, so
                     // continue processing.