diff --git a/Userland/Libraries/LibJS/Runtime/ArrayPrototype.cpp b/Userland/Libraries/LibJS/Runtime/ArrayPrototype.cpp
index 378657b773..595571d801 100644
--- a/Userland/Libraries/LibJS/Runtime/ArrayPrototype.cpp
+++ b/Userland/Libraries/LibJS/Runtime/ArrayPrototype.cpp
@@ -531,7 +531,7 @@ JS_DEFINE_NATIVE_FUNCTION(ArrayPrototype::concat)
         if (!spreadable.is_undefined())
             return spreadable.to_boolean();
 
-        return object->is_array();
+        return val.is_array(global_object);
     };
 
     auto append_to_new_array = [&vm, &is_concat_spreadable, &new_array, &global_object, &n](Value arg) {
@@ -546,7 +546,7 @@ JS_DEFINE_NATIVE_FUNCTION(ArrayPrototype::concat)
             if (vm.exception())
                 return;
 
-            if (length + k > MAX_ARRAY_LIKE_INDEX) {
+            if (n + length > MAX_ARRAY_LIKE_INDEX) {
                 vm.throw_exception<TypeError>(global_object, ErrorType::ArrayMaxSize);
                 return;
             }
diff --git a/Userland/Libraries/LibJS/Tests/builtins/Array/Array.prototype.concat.js b/Userland/Libraries/LibJS/Tests/builtins/Array/Array.prototype.concat.js
index c19b0ec13f..1681dab46c 100644
--- a/Userland/Libraries/LibJS/Tests/builtins/Array/Array.prototype.concat.js
+++ b/Userland/Libraries/LibJS/Tests/builtins/Array/Array.prototype.concat.js
@@ -40,4 +40,14 @@ describe("normal behavior", () => {
         expect(concatenated[4]).toBe(1);
         expect(concatenated[5]).toEqual([2, 3]);
     });
+
+    test("Proxy is concatenated as array", () => {
+        var proxy = new Proxy([9, 8], {});
+        var concatenated = array.concat(proxy);
+        expect(array).toHaveLength(1);
+        expect(concatenated).toHaveLength(3);
+        expect(concatenated[0]).toBe("hello");
+        expect(concatenated[1]).toBe(9);
+        expect(concatenated[2]).toBe(8);
+    });
 });