Unity-Technologies/UnityCsReference · error · ArgumentNullException

Parameter prefabAssetGameObject is null

Error message

Parameter prefabAssetGameObject is null

What it means

SavePrefabAsset requires a non-null asset GameObject. The method dereferences the asset immediately (type checks, path lookup), so a null argument throws ArgumentNullException. Note the message refers to 'prefabAssetGameObject' (the internal parameter name) rather than the public 'asset' name.

Source

Thrown at Editor/Mono/Prefabs/PrefabUtility.cs:2018

            }

            var empty = new GameObject("EmptyPrefab");
            bool success;
            var assetObject = SaveAsPrefabAsset(empty, path, out success);
            Object.DestroyImmediate(empty);
            return PrefabUtility.GetPrefabObject(assetObject);
        }

        public static GameObject SavePrefabAsset(GameObject asset)
        {
            bool savedSuccesfully;
            return SavePrefabAsset(asset, out savedSuccesfully);
        }

        public static GameObject SavePrefabAsset(GameObject asset, out bool savedSuccessfully)
        {
            if (asset == null)
                throw new ArgumentNullException("Parameter prefabAssetGameObject is null");

            // Include model check even though models are also immutable, since we can give a more clear exception message.
            if (IsPartOfModelPrefab(asset))
                throw new ArgumentException("Can't save a Model Prefab");

            if (IsPartOfImmutablePrefab(asset))
                throw new ArgumentException("Can't save an immutable Prefab");

            string path = AssetDatabase.GetAssetPath(asset);
            if (String.IsNullOrEmpty(path))
                throw new ArgumentException("Can't save a Prefab instance");

            var root = asset.transform.root.gameObject;
            if (root != asset)
                throw new ArgumentException("GameObject to save Prefab from must be a Prefab root");

            return SavePrefabAsset_Internal(root, out savedSuccessfully);
        }

View on GitHub (pinned to 225b0fbdb5)

Solutions

1. Null-check the asset before calling.
2. Resolve the asset from a known source (AssetDatabase.LoadAssetAtPath) and verify before saving.
3. Guard with a clear editor message when the resolved asset is null.

Example fix

// before
PrefabUtility.SavePrefabAsset(asset);

// after
if (asset != null)
    PrefabUtility.SavePrefabAsset(asset);

if (asset == null) return;

static bool IsPrefabAsset(GameObject asset) => asset != null;

When it happens

Trigger: Calling PrefabUtility.SavePrefabAsset(null) or SavePrefabAsset(null, out _); passing a reference that resolved to null.

Common situations: Resolving the prefab asset via a query that returned null (e.g., selection active object was not a prefab); field never initialized before the save call.

Related errors

• Cannot apply added GameObjects. GameObjects array is null.
• Cannot revert added GameObject. GameObject is null.
• guid
• prefabInstance
• Provided GameObject is not a Prefab instance

AI-assisted analysis of Unity-Technologies/UnityCsReference@225b0fbdb5 (2026-08-13). Data as JSON: /api/errors/d014c1c246586062. Report an issue: GitHub.