diff --git a/Tests/LibWeb/Text/expected/window-proxy-numeric-own-property.txt b/Tests/LibWeb/Text/expected/window-proxy-numeric-own-property.txt
new file mode 100644
index 0000000000..d20b210c6c
--- /dev/null
+++ b/Tests/LibWeb/Text/expected/window-proxy-numeric-own-property.txt
@@ -0,0 +1,3 @@
+  true
+[object Window]
+false
diff --git a/Tests/LibWeb/Text/input/window-proxy-numeric-own-property.html b/Tests/LibWeb/Text/input/window-proxy-numeric-own-property.html
new file mode 100644
index 0000000000..ae61ba3cad
--- /dev/null
+++ b/Tests/LibWeb/Text/input/window-proxy-numeric-own-property.html
@@ -0,0 +1,9 @@
+<script src="include.js"></script>
+<iframe src="about:blank"></iframe>
+<script>
+    test(() => {
+        println(0 in window)
+        println(window[0])
+        println(1 in window)
+    });
+</script>
diff --git a/Userland/Libraries/LibWeb/HTML/WindowProxy.cpp b/Userland/Libraries/LibWeb/HTML/WindowProxy.cpp
index 4554c0157a..3887b75f78 100644
--- a/Userland/Libraries/LibWeb/HTML/WindowProxy.cpp
+++ b/Userland/Libraries/LibWeb/HTML/WindowProxy.cpp
@@ -80,7 +80,12 @@ JS::ThrowCompletionOr<Optional<JS::PropertyDescriptor>> WindowProxy::internal_ge
 
         // 4. If maxProperties is greater than 0 and index is less than maxProperties, then set value to the WindowProxy object of the indexth document-tree child browsing context of W's browsing context, sorted in the order that their browsing context container elements were most recently inserted into W's associated Document, the WindowProxy object of the most recently inserted browsing context container's nested browsing context being last.
         if (max_properties > 0 && index < max_properties) {
-            // FIXME: Implement this.
+            JS::MarkedVector<BrowsingContext*> browsing_contexts { vm.heap() };
+            m_window->browsing_context()->for_each_child([&](BrowsingContext& child) {
+                if (child.container() && child.container()->in_a_document_tree())
+                    browsing_contexts.append(&child);
+            });
+            value = JS::Value(browsing_contexts[index]->window_proxy());
         }
 
         // 5. If value is undefined, then: