diff --git a/docs/contract.md b/docs/contract.md
index a461f70a..a42c5d6d 100644
--- a/docs/contract.md
+++ b/docs/contract.md
@@ -321,7 +321,10 @@ masks/                         masks folder, outside the experiment folder
 
 The optional mask step reads every image's masks, one image at a time, and describes each
 mask type and every ordered pair of types over the whole experiment. Pieces are counted with
-4-connectivity; label 0 is background. Every conclusion is a suggestion with its evidence:
+4-connectivity; label 0 is background and every object has a positive label: a mask holding
+a negative value (possible in a signed integer type, which the header check accepts) raises
+an error when its pixels are read, and no report is written, so the background or a negative
+value is never counted as an object. Every conclusion is a suggestion with its evidence:
 the user names the masks and confirms or corrects the relationships.
 
 | Kind of mask | Rule |
@@ -384,5 +387,6 @@ The report is UTF-8 JSON (two-space indent, LF), written atomically to the outpu
 | `versions` | As in the manifest (§9); information only. |
 
 The step is the command `masks` and the functions `find_masks` and `scan_masks`; it reads
-each image's masks once and holds only one image's masks in memory. A mask that is not 2D
-when its pixels are read raises an error.
+each image's masks once and holds only one image's masks in memory. A mask that cannot be read
+as the planes its header gave, or holds negative labels, when its pixels are read raises an
+error.
diff --git a/docs/user_guide.md b/docs/user_guide.md
index 4bb6ef55..7957a4f1 100644
--- a/docs/user_guide.md
+++ b/docs/user_guide.md
@@ -427,7 +427,9 @@ mask that segment the same objects are reported as such. These are suggestions f
 pixels, not decisions: you name each kind of mask and check the relationships in the bricks
 that use them. The report is written to `mask_relationships.json` in the output folder
 (§6). The command takes `--output`, `--exclude` and `--log` like the others, and exits with
-status 1 when an error blocks it or when no masks are found.
+status 1 when an error blocks it or when no masks are found. Label 0 is the background and
+objects have positive labels: a mask with a negative value stops the command with a message
+naming the mask, and no report is written.
 
 The rules behind each suggestion use thresholds you can change, for example
 `--inside-object-share 0.8` to call a kind of mask "inside" another when 80% of its objects
@@ -654,7 +656,8 @@ write_mask_report(report, "/data/exp1_MorphoBricks_Preprocessing/mask_relationsh
 
 `scan_masks(..., thresholds={"inside_object_share": 0.8})` changes a threshold (the others
 keep their defaults, `DEFAULT_THRESHOLDS`). `preflight(..., ome_zarr_labels=True)` uses the
-label images of OME-Zarr images without a masks folder; `read_mask` reads one mask's pixels;
+label images of OME-Zarr images without a masks folder; `read_mask` reads one mask's pixels
+(a `ValueError` for negative labels, as in `piece_counts`, `pair_counts` and `scan_masks`);
 `pair_counts` and `piece_counts` give the counts behind the report for one pair of masks or
 one mask.
 
diff --git a/src/morphobricks_preprocessing/cli.py b/src/morphobricks_preprocessing/cli.py
index aec77490..7316e92c 100644
--- a/src/morphobricks_preprocessing/cli.py
+++ b/src/morphobricks_preprocessing/cli.py
@@ -403,7 +403,11 @@ def progress(done: int, total: int, path: str) -> None:
             print(f"[{done + 1}/{total}] {path}", flush=True)
 
     print()
-    report = scan_masks(pre.masks, pre.discovery.root, progress=progress, thresholds=thresholds)
+    try:
+        report = scan_masks(pre.masks, pre.discovery.root, progress=progress, thresholds=thresholds)
+    except ValueError as exc:  # masks whose pixels cannot be used (planes, negative labels)
+        print(f"The masks cannot be used: {exc}")
+        return 1
     output = Path(args.output) if args.output else default_output_folder(pre.discovery.root)
     path = write_mask_report(report, output / MASK_REPORT_FILENAME)
     print()
diff --git a/src/morphobricks_preprocessing/mask_scan.py b/src/morphobricks_preprocessing/mask_scan.py
index 1a286839..d21a8d26 100644
--- a/src/morphobricks_preprocessing/mask_scan.py
+++ b/src/morphobricks_preprocessing/mask_scan.py
@@ -54,7 +54,7 @@
 from scipy import ndimage
 
 from .manifest import package_versions
-from .masks import MaskInventory, read_mask
+from .masks import MaskInventory, check_labels, read_mask
 
 logger = logging.getLogger(__name__)
 
@@ -247,15 +247,22 @@ def piece_counts(mask: np.ndarray) -> tuple[np.ndarray, np.ndarray]:
     """Return the labels of a 2D mask and the number of 4-connected pieces of each.
 
     Two pixels are in one piece when they carry the same label and touch along an edge.
+    Label 0 is the background.
 
     Returns
     -------
     tuple of numpy.ndarray
         The sorted nonzero labels, and the number of pieces of each.
+
+    Raises
+    ------
+    ValueError
+        If the mask holds negative labels (:func:`.masks.check_labels`).
     """
+    check_labels(mask)
     values, dense = np.unique(mask, return_inverse=True)
     dense = dense.reshape(mask.shape)
-    if values.size and values[0] == 0:
+    if values.size and values[0] == 0:  # labels are never negative, so 0 comes first
         values = values[1:]
     else:
         dense = dense + 1  # no background: every value is a label
@@ -273,8 +280,15 @@ def pair_counts(
 
     Both are 2D label images of one position; label 0 is background. ``thresholds`` changes
     the default ``inside_pixel_share`` and ``same_objects_iou`` (:func:`mask_thresholds`).
+
+    Raises
+    ------
+    ValueError
+        If either mask holds negative labels (:func:`.masks.check_labels`).
     """
     limits = mask_thresholds(thresholds)
+    check_labels(child, "the child mask")
+    check_labels(parent, "the parent mask")
     counts = dict.fromkeys(_COUNTS, 0)
     on = child != 0
     child_labels, child_of = np.unique(child[on], return_inverse=True)
@@ -349,7 +363,7 @@ def scan_masks(
     ValueError
         For an invalid threshold, before any mask is read; for an image whose masks have
         different depth slices or time points; for a mask that cannot be read as the planes
-        its header gave (:func:`.masks.read_mask`).
+        its header gave, or holds negative labels (:func:`.masks.read_mask`).
     """
     limits = mask_thresholds(thresholds)
     root = Path(experiment_root).resolve()
diff --git a/src/morphobricks_preprocessing/masks.py b/src/morphobricks_preprocessing/masks.py
index c87be4fb..fbb6e262 100644
--- a/src/morphobricks_preprocessing/masks.py
+++ b/src/morphobricks_preprocessing/masks.py
@@ -233,7 +233,8 @@ def read_mask(
     ------
     ValueError
         If the mask's dimensions are not the ones its header gave (``mask.axes``), or ``z``
-        or ``t`` is missing or outside the mask.
+        or ``t`` is missing or outside the mask, or the plane holds negative labels
+        (label 0 is the background and objects are positive, contract §15).
     """
     wanted = {"Z": z, "T": t}
     for axis, index in wanted.items():
@@ -258,6 +259,7 @@ def read_mask(
         axes = "".join(a for a, s in zip(labels.axes, selection, strict=True) if s == slice(None))
     if axes != "YX" or (mask.axes and pixels.shape != tuple(_yx(mask.axes, mask.shape))):
         raise ValueError(f"the mask {mask.path} is not the 2D plane its header gave ({axes})")
+    check_labels(pixels, mask.path)
     return pixels
 
 
@@ -298,6 +300,25 @@ def _yx(axes: str, shape: tuple[int, ...]) -> tuple[int, int]:
     return shape[axes.index("Y")], shape[axes.index("X")]
 
 
+def check_labels(pixels: np.ndarray, name: str = "the mask") -> None:
+    """Refuse a mask holding negative labels.
+
+    Label 0 is the background and every object has a positive label (contract §15). A
+    negative value is neither, so it is refused rather than read as an object (which would
+    also make the background one).
+
+    Raises
+    ------
+    ValueError
+        If any pixel is below 0.
+    """
+    if pixels.dtype.kind == "i" and pixels.size and pixels.min() < 0:
+        raise ValueError(
+            f"{name} holds negative labels (smallest {pixels.min()}); label 0 is the "
+            "background and objects must have positive labels."
+        )
+
+
 def _folder_issues(folder: Path, experiment_root: Path) -> list[ValidationIssue]:
     if not folder.is_dir():
         return [
diff --git a/tests/test_mask_scan.py b/tests/test_mask_scan.py
index 3c206f86..9a0e7bd7 100644
--- a/tests/test_mask_scan.py
+++ b/tests/test_mask_scan.py
@@ -350,3 +350,41 @@ def test_masks_with_time_points_are_scanned_plane_by_plane(tmp_path):
     assert [(p.child, p.parent, dict(p.evidence)) for p in report.pairs] == [
         (p.child, p.parent, dict(p.evidence)) for p in flat.pairs
     ]
+
+
+# Negative labels (issue #35) ----------------------------------------------------------
+
+
+def test_negative_labels_are_refused_never_read_as_objects(tmp_path):
+    signed = np.zeros(SHAPE, dtype=np.int16)
+    signed[2:8, 2:8] = -1
+    signed[20:30, 20:30] = 3
+    with pytest.raises(ValueError, match="negative labels"):
+        piece_counts(signed)
+    with pytest.raises(ValueError, match="negative labels"):
+        pair_counts(signed, boxes(CELLS))
+    with pytest.raises(ValueError, match="negative labels"):
+        pair_counts(boxes(NUCLEI), signed)
+    # Signed masks without negative labels keep 0 as the background.
+    labels, _ = piece_counts(np.abs(signed))
+    assert labels.tolist() == [1, 3]
+    root, folder = write_experiment(tmp_path, {"Cells": boxes(CELLS), "Signed": signed})
+    inventory = preflight(root, masks_folder=folder).masks
+    assert inventory.ok  # headers only: an int16 mask is fine
+    with pytest.raises(ValueError, match="Signed/ctrl/a.tif holds negative labels"):
+        scan_masks(inventory, root)
+
+
+def test_negative_labels_in_a_later_time_point_are_refused(tmp_path):
+    # #36 reads masks plane by plane; #35's check must cover every plane, not only the first.
+    frames = hierarchy_masks()
+    signed = np.stack([frames["Cells"].astype(np.int16)] * 3)
+    signed[2, 0, 0] = -1
+    root, folder = tmp_path / "exp", tmp_path / "masks"
+    write_data(root / "ctrl" / "a.tif", np.zeros((3, *SHAPE), dtype=np.uint8), "TYX")
+    write_data(folder / "Cells" / "ctrl" / "a.tif", signed, "TYX")
+    write_data(folder / "Nuclei" / "ctrl" / "a.tif", np.stack([frames["Nuclei"]] * 3), "TYX")
+    inventory = preflight(root, masks_folder=folder).masks
+    assert inventory.ok, inventory.issues
+    with pytest.raises(ValueError, match="Cells/ctrl/a.tif holds negative labels"):
+        scan_masks(inventory, root)
diff --git a/tests/test_masks.py b/tests/test_masks.py
index 8ff44f7c..76bbc816 100644
--- a/tests/test_masks.py
+++ b/tests/test_masks.py
@@ -435,3 +435,13 @@ def test_label_images_with_time_points(tmp_path):
         ome_zarr_labels=True,
     )
     assert result.written, result.issues
+
+
+def test_masks_command_refuses_negative_labels(experiment, masks, tmp_path, capsys):
+    signed = label_image(0).astype(np.int32)
+    signed[0, 0] = -5
+    write_data(masks / "Cells" / "ctrl" / "rep1" / "a.tif", signed)
+    out = tmp_path / "out"
+    assert main(["masks", str(experiment), "--masks", str(masks), "--output", str(out)]) == 1
+    assert "Cells/ctrl/rep1/a.tif holds negative labels (smallest -5)" in capsys.readouterr().out
+    assert not (out / "mask_relationships.json").exists()