diff --git a/toolkit/components/prompts/src/Prompter.jsm b/toolkit/components/prompts/src/Prompter.jsm
index 3c968827712a..cbc13552179a 100644
--- a/toolkit/components/prompts/src/Prompter.jsm
+++ b/toolkit/components/prompts/src/Prompter.jsm
@@ -450,7 +450,7 @@ function openModalWindow(domWin, uri, args) {
   // will do that. Similarly for enterModalState / leaveModalState.
 
   Services.ww.openWindow(
-    domWin.docShell.rootTreeItem.domWindow,
+    domWin && domWin.docShell.rootTreeItem.domWindow,
     uri,
     "_blank",
     "centerscreen,chrome,modal,titlebar",