What causes "Expected parentIndices[0] to be -1, got {parentIndices[0]}" in Unity-Technologies/ml-agents?

Set parentIndices[0] to -1 so the first pose is the root of the hierarchy.

Unity-Technologies/ml-agents · error · UnityAgentsException

Expected parentIndices[0] to be -1, got {parentIndices[0]}

Error message

Expected parentIndices[0] to be -1, got {parentIndices[0]}

What it means

PoseExtractor.Setup validates (in DEBUG builds only) that parentIndices[0] == -1, i.e. the first entry must be the root of the pose hierarchy with no parent. If it isn't, UnityAgentsException is thrown. This guards the invariant that the hierarchy has exactly one well-defined root at index 0.

Source

Thrown at com.unity.ml-agents/Runtime/Sensors/PoseExtractor.cs:185

        /// </summary>
        /// <param name="index">The index of the pose to check.</param>
        /// <returns>True if the pose is enabled; otherwise, false.</returns>
        public bool IsPoseEnabled(int index)
        {
            return m_PoseEnabled[index];
        }

        /// <summary>
        /// Initialize with the mapping of parent indices.
        /// The 0th element is assumed to be -1, indicating that it's the root.
        /// </summary>
        /// <param name="parentIndices">An array mapping each pose to its parent index. The root should be -1.</param>
        protected void Setup(int[] parentIndices)
        {
#if DEBUG
            if (parentIndices[0] != -1)
            {
                throw new UnityAgentsException($"Expected parentIndices[0] to be -1, got {parentIndices[0]}");
            }
#endif
            m_ParentIndices = parentIndices;
            var numPoses = parentIndices.Length;
            m_ModelSpacePoses = new Pose[numPoses];
            m_LocalSpacePoses = new Pose[numPoses];

            m_ModelSpaceLinearVelocities = new Vector3[numPoses];
            m_LocalSpaceLinearVelocities = new Vector3[numPoses];

            m_PoseEnabled = new bool[numPoses];
            // All poses are enabled by default. Generally we'll want to disable the root though.
            for (var i = 0; i < numPoses; i++)
            {
                m_PoseEnabled[i] = true;
            }
        }

View on GitHub (pinned to 3ecb446f75)

Solutions

Set parentIndices[0] to -1 so the first pose is the root of the hierarchy.
Rebuild the parent map by walking the bone hierarchy from the root outward so the root lands first.
Note this check only fires in DEBUG builds — validate the array yourself in release builds before calling Setup.

Example fix

// before
Setup(new[] { 0, 0, 1 }); // parentIndices[0] = 0, not -1
// after
Setup(new[] { -1, 0, 1 }); // root first, then children

Defensive patterns

Strategy: validation

Validate before calling

if (parentIndices == null || parentIndices.Length == 0 || parentIndices[0] != -1)
    throw new ArgumentException("parentIndices[0] must be -1 (root)");

Type guard

static bool IsValidParentIndices(int[] p) => p != null && p.Length > 0 && p[0] == -1;

Try / catch

try { Setup(parentIndices); } catch (UnityAgentsException e) { Debug.LogError($"Bad parent map: {e.Message}"); }

Prevention

Put the hierarchy root at index 0 with parent -1
Generate parent arrays by walking bones root-first
Remember the check only runs in DEBUG builds — validate in release too

When it happens

Trigger: Calling Setup(int[] parentIndices) where the first element is not -1 — e.g. passing an all-positive array, an array whose root is at another index, or a misbuilt parent map from a custom rig.

Common situations: Building a custom PoseExtractor for a custom character rig and forgetting to place the root at index 0; generating parent indices programmatically from a bone hierarchy and starting from a non-root bone; copying an array in the wrong order.

Related errors

AI-assisted analysis of Unity-Technologies/ml-agents@3ecb446f75 (2026-09-02). Data as JSON: /api/errors/28b3927d4e70388f. Report an issue: GitHub.