roboflow/supervision · error · ValueError

ConfusionMatrix can only be calculated for Detections with c

Error message

ConfusionMatrix can only be calculated for Detections with class_id

What it means

Error "ConfusionMatrix can only be calculated for Detections with class_id" thrown in roboflow/supervision.

Source

Thrown at src/supervision/metrics/detection.py:134

        (1, 6)
        >>> obb_coords = np.array([[0, 0, 10, 0, 10, 10, 0, 10]], dtype=np.float32)
        >>> det_obb = sv.Detections(
        ...     xyxy=np.array([[0, 0, 10, 10]], dtype=np.float32),
        ...     class_id=np.array([0]),
        ...     data={ORIENTED_BOX_COORDINATES: obb_coords},
        ... )
        >>> tensor_obb = detections_to_tensor(
        ...     det_obb, metric_target=MetricTarget.ORIENTED_BOUNDING_BOXES
        ... )
        >>> tensor_obb.shape
        (1, 9)

        ```
    """
    _assert_supported_target(metric_target)

    if detections.class_id is None:
        raise ValueError(
            "ConfusionMatrix can only be calculated for Detections with class_id"
        )

    box_data: npt.NDArray[np.float32]
    if metric_target == MetricTarget.ORIENTED_BOUNDING_BOXES:
        obb = detections.data.get(ORIENTED_BOX_COORDINATES)
        if obb is None:
            if len(detections) > 0:
                raise ValueError(
                    "ORIENTED_BOUNDING_BOXES requested, but "
                    f"{ORIENTED_BOX_COORDINATES} is missing from detections.data"
                )
            box_data = np.empty((0, 8), dtype=np.float32)
        else:
            obb_arr = np.asarray(obb, dtype=np.float32)
            # Normalize (N, 4, 2) → (N, 8) as produced by from_ultralytics.
            if obb_arr.ndim == 3 and obb_arr.shape[1:] == (4, 2):
                obb_arr = obb_arr.reshape(-1, 8)

View on GitHub (pinned to 7f254d9784)

Solutions

1. Set class_id on the Detections before computing the confusion matrix.
2. Derive class_id from your model output and pass it to the Detections constructor.

When it happens

Trigger: Thrown at src/supervision/metrics/detection.py:134 when the library encounters an invalid state.

Common situations: See trigger scenarios.

AI-assisted analysis of roboflow/supervision@7f254d9784 (2026-08-15). Data as JSON: /api/errors/6d9f31686708c873. Report an issue: GitHub.