--- a/browser/components/extensions/test/browser/browser_ext_getViews.js
+++ b/browser/components/extensions/test/browser/browser_ext_getViews.js
@@ -126,16 +126,17 @@ add_task(async function() {
 
   async function checkViewsWithFilter(filter, expectedCount) {
     extension.sendMessage("background-getViews-with-filter", filter);
     let count = await extension.awaitMessage("getViews-count");
     is(count, expectedCount, `count for ${JSON.stringify(filter)} correct`);
   }
 
   await checkViews("background", 0, 0, 0);
+  await checkViewsWithFilter({windowId: -1}, 1);
   await checkViewsWithFilter({tabId: -1}, 1);
 
   let tabId1 = await openTab(winId1);
 
   await checkViews("background", 1, 0, 0);
   await checkViews("tab", 1, 0, 1);
   await checkViewsWithFilter({windowId: winId1}, 1);
   await checkViewsWithFilter({tabId: tabId1}, 1);

--- a/toolkit/components/extensions/ExtensionPageChild.jsm
+++ b/toolkit/components/extensions/ExtensionPageChild.jsm
@@ -148,16 +148,17 @@ class ExtensionBaseContextChild extends 
     return this.contentWindow.document.nodePrincipal;
   }
 
   get windowId() {
     if (["tab", "popup", "sidebar"].includes(this.viewType)) {
       let globalView = ExtensionPageChild.contentGlobals.get(this.messageManager);
       return globalView ? globalView.windowId : -1;
     }
+    return -1;
   }
 
   get tabId() {
     // Will be overwritten in the constructor if necessary.
     return -1;
   }
 
   // Called when the extension shuts down.