Unity-Technologies/UnityCsReference · error · ArgumentNullException

Value cannot be null. (Parameter 'prefabAsset')

Error message

Value cannot be null. (Parameter 'prefabAsset')

What it means

Thrown by ThrowIfInvalidAssetForReplacePrefabInstance when prefabAsset is null. This guard ensures a valid Prefab asset is provided before attempting to replace a Prefab instance with it.

Source

Thrown at Editor/Mono/Prefabs/PrefabUtility.cs:2222

        {
            return InstantiatePrefab_internal(assetComponentOrGameObject, EditorSceneManager.GetTargetSceneForNewGameObjects(), null);
        }

        // Instantiates the given prefab in a given scene
        public static Object InstantiatePrefab(Object assetComponentOrGameObject, Scene destinationScene)
        {
            return InstantiatePrefab_internal(assetComponentOrGameObject, destinationScene, null);
        }

        public static Object InstantiatePrefab(Object assetComponentOrGameObject, Transform parent)
        {
            return InstantiatePrefab_internal(assetComponentOrGameObject, EditorSceneManager.GetTargetSceneForNewGameObjects(), parent);
        }

        internal static void ThrowIfInvalidAssetForReplacePrefabInstance(GameObject prefabAsset, InteractionMode action)
        {
            if (prefabAsset == null)
                throw new ArgumentNullException(nameof(prefabAsset));

            if (!EditorUtility.IsPersistent(prefabAsset))
                throw new ArgumentException("Input Prefab asset is not an asset object. Input asset: " + prefabAsset.name, nameof(prefabAsset));

            var assetPath = AssetDatabase.GetAssetPath(prefabAsset);
            if (assetPath.StartsWith("Library/"))
                throw new InvalidOperationException(string.Format("Cannot replace the Prefab instance since the Prefab Asset is invalid for instance replacement. Prefab Asset path: " + assetPath));

            // Recording undo does not handle missing scripts
            var gameObjectsWithInvalidScript = FindGameObjectsWithInvalidComponent(prefabAsset);
            if (action == InteractionMode.UserAction && gameObjectsWithInvalidScript.Count > 0)
                throw new InvalidOperationException(string.Format($"Cannot replace the Prefab instance with the Prefab Asset '{AssetDatabase.GetAssetPath(prefabAsset)}' because it has a missing script. GameObject '{gameObjectsWithInvalidScript[0].name}' in the Prefab Asset has a missing script."));
        }

        internal static void ThrowIfInvalidArgumentsForReplacePrefabInstance(GameObject prefabInstanceRoot, GameObject prefabAssetRoot, bool checkValidAsset, InteractionMode mode)
        {
            if (prefabInstanceRoot == null)
                throw new ArgumentNullException(nameof(prefabInstanceRoot));

View on GitHub (pinned to 225b0fbdb5)

Solutions

1. Null-check prefabAsset before calling the replace API.
2. Verify the asset path with AssetDatabase.LoadAssetAtPath and check for null before proceeding.
3. Re-resolve asset references after AssetDatabase.Refresh or domain reload.

Example fix

// before
GameObject asset = AssetDatabase.LoadAssetAtPath<GameObject>(badPath);
ThrowIfInvalidAssetForReplacePrefabInstance(asset, mode);
// after
GameObject asset = AssetDatabase.LoadAssetAtPath<GameObject>(path);
if (asset == null) { Debug.LogError("Asset not found: " + path); return; }
ThrowIfInvalidAssetForReplacePrefabInstance(asset, mode);

if (prefabAsset == null)
{ Debug.LogError("Prefab asset is null; check the asset path."); return; }
PrefabUtility.ThrowIfInvalidAssetForReplacePrefabInstance(prefabAsset, mode);

static bool IsValidReplacementAsset(GameObject asset) =>
    asset != null && EditorUtility.IsPersistent(asset);

When it happens

Trigger: Calling ReplacePrefabInstance (internal) with a null prefabAsset argument, typically from a failed AssetDatabase lookup or unassigned reference.

Common situations: AssetDatabase.LoadAssetAtPath returned null due to wrong path or missing asset. Serialized Prefab reference lost after domain reload. Editor script assumes an asset exists that was renamed or deleted.

Related errors

• prefabAssetRoot
• Parameter root is null
• Value cannot be null. (Parameter 'instance')
• Input Prefab asset is not an asset object. Input asset: {pre
• Cannot replace the Prefab instance since the Prefab Asset is

AI-assisted analysis of Unity-Technologies/UnityCsReference@225b0fbdb5 (2026-08-13). Data as JSON: /api/errors/c092eaa7c7b6cb60. Report an issue: GitHub.