diff --git a/app/scripts/fetch_xulrunner b/app/scripts/fetch_xulrunner
index 330227bdde..47a4099cb6 100755
--- a/app/scripts/fetch_xulrunner
+++ b/app/scripts/fetch_xulrunner
@@ -197,6 +197,8 @@ function modify_omni {
 	# Accept zotero@chnm.gmu.edu for target application to allow Zotero 6 plugins to remain
 	# installed in Zotero 7
 	replace_line "if \(targetApp.id == Services.appinfo.ID\) \{" "if (targetApp.id == 'zotero\@chnm.gmu.edu') targetApp.id = '$APP_ID'; if (targetApp.id == Services.appinfo.ID) {" modules/addons/XPIDatabase.jsm
+	# Fix addon.id is undefined with built-in theme addons, anyway we don't support theme addons
+	replace_line 'addon.type === "theme"' 'false' modules/addons/XPIDatabase.jsm
 	
 	# For updates, look for applications.zotero instead of applications.gecko in manifest.json and
 	# use the app id and version for strict_min_version/strict_max_version comparisons