diff --git a/Userland/Libraries/LibJS/Runtime/ErrorTypes.h b/Userland/Libraries/LibJS/Runtime/ErrorTypes.h
index bd91b3d827..09a1ffacfc 100644
--- a/Userland/Libraries/LibJS/Runtime/ErrorTypes.h
+++ b/Userland/Libraries/LibJS/Runtime/ErrorTypes.h
@@ -152,6 +152,12 @@
                                              "the type of each result list element is either String or Symbol")                         \
     M(ProxyOwnPropertyKeysDuplicates, "Proxy handler's ownKeys trap violates invariant: "                                               \
                                       "the result list may not contain duplicate elements")                                             \
+    M(ProxyOwnPropertyKeysSkippedNonconfigurableProperty, "Proxy handler's ownKeys trap violates invariant: "                           \
+                                                          "cannot skip non-configurable property '{}'")                                 \
+    M(ProxyOwnPropertyKeysNonExtensibleSkippedProperty, "Proxy handler's ownKeys trap violates invariant: "                             \
+                                                        "cannot skip property '{}' of non-extensible object")                           \
+    M(ProxyOwnPropertyKeysNonExtensibleNewProperty, "Proxy handler's ownKeys trap violates invariant: "                                 \
+                                                    "cannot report new property '{}' of non-extensible object")                         \
     M(ProxyPreventExtensionsReturn, "Proxy handler's preventExtensions trap violates "                                                  \
                                     "invariant: cannot return true if the target object is extensible")                                 \
     M(ProxyRevoked, "An operation was performed on a revoked Proxy object")                                                             \
diff --git a/Userland/Libraries/LibJS/Runtime/ProxyObject.cpp b/Userland/Libraries/LibJS/Runtime/ProxyObject.cpp
index 2a47c2909f..1390ebb6c9 100644
--- a/Userland/Libraries/LibJS/Runtime/ProxyObject.cpp
+++ b/Userland/Libraries/LibJS/Runtime/ProxyObject.cpp
@@ -728,7 +728,7 @@ ThrowCompletionOr<MarkedValueList> ProxyObject::internal_own_property_keys() con
     for (auto& key : target_nonconfigurable_keys) {
         // a. If key is not an element of uncheckedResultKeys, throw a TypeError exception.
         if (!unchecked_result_keys.contains_slow(key))
-            return vm.throw_completion<TypeError>(global_object, ErrorType::FixmeAddAnErrorString);
+            return vm.throw_completion<TypeError>(global_object, ErrorType::ProxyOwnPropertyKeysSkippedNonconfigurableProperty, key.to_string_without_side_effects());
 
         // b. Remove key from uncheckedResultKeys.
         unchecked_result_keys.remove_first_matching([&](auto& value) {
@@ -744,7 +744,7 @@ ThrowCompletionOr<MarkedValueList> ProxyObject::internal_own_property_keys() con
     for (auto& key : target_configurable_keys) {
         // a. If key is not an element of uncheckedResultKeys, throw a TypeError exception.
         if (!unchecked_result_keys.contains_slow(key))
-            return vm.throw_completion<TypeError>(global_object, ErrorType::FixmeAddAnErrorString);
+            return vm.throw_completion<TypeError>(global_object, ErrorType::ProxyOwnPropertyKeysNonExtensibleSkippedProperty, key.to_string_without_side_effects());
 
         // b. Remove key from uncheckedResultKeys.
         unchecked_result_keys.remove_first_matching([&](auto& value) {
@@ -754,7 +754,7 @@ ThrowCompletionOr<MarkedValueList> ProxyObject::internal_own_property_keys() con
 
     // 22. If uncheckedResultKeys is not empty, throw a TypeError exception.
     if (!unchecked_result_keys.is_empty())
-        return vm.throw_completion<TypeError>(global_object, ErrorType::FixmeAddAnErrorString);
+        return vm.throw_completion<TypeError>(global_object, ErrorType::ProxyOwnPropertyKeysNonExtensibleNewProperty, unchecked_result_keys[0].to_string_without_side_effects());
 
     // 23. Return trapResult.
     return { move(trap_result) };
diff --git a/Userland/Libraries/LibJS/Tests/builtins/Proxy/Proxy.handler-ownKeys.js b/Userland/Libraries/LibJS/Tests/builtins/Proxy/Proxy.handler-ownKeys.js
new file mode 100644
index 0000000000..6dba73c337
--- /dev/null
+++ b/Userland/Libraries/LibJS/Tests/builtins/Proxy/Proxy.handler-ownKeys.js
@@ -0,0 +1,56 @@
+// TODO: Add "[[OwnPropertyKeys]] trap normal behavior" tests
+
+describe("[[OwnPropertyKeys]] invariants", () => {
+    // TODO: Add tests for other [[OwnPropertyKeys]] trap invariants
+
+    test("cannot report new property of non-extensible object", () => {
+        const target = Object.preventExtensions({});
+        const handler = {
+            ownKeys() {
+                return ["foo", "bar", "baz"];
+            },
+        };
+        const proxy = new Proxy(target, handler);
+
+        expect(() => {
+            Reflect.ownKeys(proxy);
+        }).toThrowWithMessage(
+            TypeError,
+            "Proxy handler's ownKeys trap violates invariant: cannot report new property 'foo' of non-extensible object"
+        );
+    });
+
+    test("cannot skip property of non-extensible object", () => {
+        const target = Object.preventExtensions({ foo: null });
+        const handler = {
+            ownKeys() {
+                return ["bar", "baz"];
+            },
+        };
+        const proxy = new Proxy(target, handler);
+
+        expect(() => {
+            Reflect.ownKeys(proxy);
+        }).toThrowWithMessage(
+            TypeError,
+            "Proxy handler's ownKeys trap violates invariant: cannot skip property 'foo' of non-extensible object"
+        );
+    });
+
+    test("cannot skip non-configurable property", () => {
+        const target = Object.defineProperty({}, "foo", { configurable: false });
+        const handler = {
+            ownKeys() {
+                return ["bar", "baz"];
+            },
+        };
+        const proxy = new Proxy(target, handler);
+
+        expect(() => {
+            Reflect.ownKeys(proxy);
+        }).toThrowWithMessage(
+            TypeError,
+            "Proxy handler's ownKeys trap violates invariant: cannot skip non-configurable property 'foo'"
+        );
+    });
+});