diff --git a/plugins/computers/plugin.py b/plugins/computers/plugin.py
index cb1fd19..781f963 100644
--- a/plugins/computers/plugin.py
+++ b/plugins/computers/plugin.py
@@ -798,19 +798,26 @@ class ComputersPlugin(BasePlugin):
                 rel.isactive = False
 
         operation = self._link_marker_to_operation(
-            markerasset, machinenumber, warnings)
+            markerasset, machinenumber, pcasset, warnings)
 
         return [{'assetid': markerasset.assetid,
                  'assetnumber': markerasset.assetnumber,
                  'operationassetid': operation}]
 
-    def _link_marker_to_operation(self, markerasset, machinenumber, warnings):
+    def _link_marker_to_operation(self, markerasset, machinenumber, pcasset,
+                                  warnings):
         """Make a marker `partof` the operation whose number its PC reports.
 
         Unlike the PC-to-machine link this does NOT contest: an operation can
         hold any number of markers, which is the whole point. Moving a marker to
         another operation archives the old membership rather than deleting it,
         so where a marker used to live stays answerable.
+
+        Refuses to file a marker under the reporting PC. A PC first seen before
+        the machine-number fix was created with the machine number as its OWN
+        asset number, and that is deliberately never overwritten, so looking up
+        the number can return the PC itself. Filing the marker partof its own PC
+        would read, on the machine page, as the PC being the operation.
         """
         from shopdb.api import AssetRelationship, RelationshipType, Asset
 
@@ -834,6 +841,13 @@ class ComputersPlugin(BasePlugin):
             return None
         if operation.assetid == markerasset.assetid:
             return None
+        if pcasset is not None and operation.assetid == pcasset.assetid:
+            warnings.append(
+                'machine number {!r} is this PC\'s own asset number; marker '
+                'not filed under an operation. Rename the PC asset to its '
+                'hostname, or create the operation asset.'.format(
+                    machinenumber))
+            return None
 
         links = AssetRelationship.query.filter(
             AssetRelationship.sourceassetid == markerasset.assetid,
diff --git a/tests/test_core/test_collector_contract.py b/tests/test_core/test_collector_contract.py
index ed27ba6..71e3d25 100644
--- a/tests/test_core/test_collector_contract.py
+++ b/tests/test_core/test_collector_contract.py
@@ -1008,3 +1008,41 @@ def test_an_ordinary_pc_gets_no_marker(
                       'pctype': 'gea-shopfloor-collections'},
                 headers={'X-API-Key': collector_key})
     assert Asset.query.filter(Asset.assetnumber.like('%-PARTMARKER')).all() == []
+
+
+def test_a_marker_is_never_filed_under_its_own_pc(
+        client, db, collector_key, computer_assettype, machine_3015):
+    """A PC first seen before the machine-number fix was created with the
+    machine number as its OWN asset number, and that is never overwritten. So
+    resolving the number can return the PC itself, and filing the marker partof
+    its own PC reads on the machine page as the PC being the operation."""
+    from shopdb.core.models import Asset, AssetRelationship, RelationshipType
+    from plugins.computers.models import Computer
+
+    db.session.add(RelationshipType(relationshiptype='partof',
+                                    isdirectional=True))
+    db.session.commit()
+
+    # A legacy PC: its own asset carries the machine number, and no separate
+    # asset for the operation exists.
+    client.post('/api/collector/computers',
+                json={'hostname': 'LEGACYPC',
+                      'pctype': 'gea-shopfloor-partmarker'},
+                headers={'X-API-Key': collector_key})
+    comp = Computer.query.filter(Computer.hostname.ilike('LEGACYPC')).first()
+    comp.asset.assetnumber = '0615'
+    db.session.commit()
+
+    r = client.post('/api/collector/computers',
+                    json={'hostname': 'LEGACYPC', 'machinenumber': '0615',
+                          'pctype': 'gea-shopfloor-partmarker'},
+                    headers={'X-API-Key': collector_key})
+
+    partof = RelationshipType.query.filter_by(relationshiptype='partof').first()
+    links = AssetRelationship.query.filter_by(
+        relationshiptypeid=partof.relationshiptypeid, isactive=True).all()
+    targets = [db.session.get(Asset, rel.targetassetid).assetid
+               for rel in links]
+    assert comp.assetid not in targets, 'marker filed under its own PC'
+    assert any("own asset number" in w
+               for w in r.get_json()['data']['warnings'])