diff --git a/NIGHTLY_CHANGELOG.md b/NIGHTLY_CHANGELOG.md
index f3f3667..43259d9 100644
--- a/NIGHTLY_CHANGELOG.md
+++ b/NIGHTLY_CHANGELOG.md
@@ -2,3 +2,4 @@
 
 - Fix: on printers with multiple daisy-chained ACE units and no toolhead buffer (e.g. Kobra S1 with 2 ACE Pro), only the first unit's 4 slots were ever shown on the dashboard or synced to OrcaSlicer — the bridge silently discarded every ACE unit after the first. All units now show up correctly (Issue #95, thanks @hoovercl for the detailed report and logs)
 - Feat: the GCode browser now supports multi-select and bulk delete — click any file's checkbox to enter select mode, use "Select All" to grab everything currently visible (respecting your active search/filter), then delete the selection with one confirmation instead of one-by-one (Issue #94, thanks @Blaim)
+- Fix: **printing via OrcaSlicer "Upload and print" failed (or fed the wrong spool) when a slot below the used filament was empty** — e.g. printing with Filament 4 while slot 3 was empty. The generated AMS slot mapping inserted a placeholder pointing at the physically empty tray, which the printer rejects. Placeholders now point at a loaded tray instead. Printing with all slots full already worked and is unchanged.
diff --git a/kobrax_moonraker_bridge.py b/kobrax_moonraker_bridge.py
index 263a86c..6215f03 100644
--- a/kobrax_moonraker_bridge.py
+++ b/kobrax_moonraker_bridge.py
@@ -1762,6 +1762,14 @@ class KobraXBridge:
         max_idx = max(loaded_map.keys())
         # The printer interprets ams_box_mapping as an ordered list (entry N = TN).
         # Missing slots must be inserted as placeholders, otherwise everything shifts.
+        # A placeholder must NOT reference a physically empty tray: the printer
+        # rejects such an entry even for a tool the GCode never calls (printing
+        # Filament 4 with the slot below it empty fails; all-full works). Point
+        # gap placeholders at a definitely-loaded tray instead of the gap's own
+        # (empty) index.
+        fallback_gidx = max_idx  # highest loaded slot -> loaded + printable
+        fallback_slot = loaded_map[fallback_gidx]
+        fallback_ams = self._slot_to_print_ams_index(fallback_gidx)
         result = []
         for i in range(max_idx + 1):
             if i in loaded_map:
@@ -1776,10 +1784,10 @@ class KobraXBridge:
             else:
                 result.append({
                     "paint_index": i,
-                    "ams_index": self._slot_to_print_ams_index(i),
+                    "ams_index": fallback_ams,
                     "paint_color": [255, 255, 255, 255],
-                    "ams_color": [255, 255, 255, 255],
-                    "material_type": "PLA",
+                    "ams_color": self._slot_color_rgba(fallback_slot),
+                    "material_type": fallback_slot.get("type", "PLA"),
                 })
         return result
 
diff --git a/tests/test_auto_ams_box_mapping_empty_slot.py b/tests/test_auto_ams_box_mapping_empty_slot.py
new file mode 100644
index 0000000..7c90466
--- /dev/null
+++ b/tests/test_auto_ams_box_mapping_empty_slot.py
@@ -0,0 +1,85 @@
+"""Empty-tray placeholder bug in the OrcaSlicer "upload and print" path.
+
+Real KX1 bug (confirmed 2026-07-22): printing Filament 4 with the slot below it
+EMPTY fails; with all slots full it works. `_start_print` -> `_build_auto_ams_box_mapping`
+inserts a positional placeholder at each gap whose `ams_index` points at the
+gap's own (physically EMPTY) tray. The printer rejects a mapping entry that
+references an empty tray, even for a tool the GCode never calls.
+
+Invariant the fix must hold: EVERY entry's ams_index references a LOADED tray.
+Positional alignment (entry N = TN, from a16062f) must be preserved.
+"""
+import argparse
+import tempfile
+from unittest.mock import MagicMock
+
+from kobrax_moonraker_bridge import KobraXBridge
+
+# Real KX1 AMS state: slot 2 (idx 2) EMPTY, below the used Filament 4 (idx 3).
+AMS_SLOTS = [
+    {"global_index": 0, "box_id": -1, "status": 5, "type": "PLA",      "color": [101, 88, 177]},
+    {"global_index": 1, "box_id": -1, "status": 5, "type": "PLA SILK", "color": [239, 96, 163]},
+    {"global_index": 2, "box_id": -1, "status": 4, "type": "PLA",      "color": [223, 221, 220]},  # EMPTY
+    {"global_index": 3, "box_id": -1, "status": 5, "type": "PLA",      "color": [117, 120, 123]},
+]
+
+
+def _bridge(slots=AMS_SLOTS):
+    c = MagicMock(); c.callbacks = {}; c.connected = False
+    args = argparse.Namespace(
+        printer_ip="", mqtt_port=9883, username="", password="",
+        mode_id="20030", device_id="", host="127.0.0.1", port=7125,
+        data_dir=tempfile.mkdtemp(prefix="kxauto-"),
+    )
+    b = KobraXBridge(c, args=args)
+    b._filament_mode = "toolhead"
+    b._ams_slots = [dict(s) for s in slots]
+    return b
+
+
+def _loaded_ams_indices(b):
+    return {b._slot_to_print_ams_index(int(s["global_index"]))
+            for s in b._ams_slots if s["status"] == 5}
+
+
+def test_no_entry_points_at_an_empty_tray_when_only_used_slot_loaded():
+    """_start_print filters loaded to the used slot only: loaded=[(3, slot3)].
+    Positions 0..2 are placeholders and must NOT reference empty tray idx 2."""
+    b = _bridge()
+    loaded = [(3, b._ams_slots[3])]
+
+    mapping = b._build_auto_ams_box_mapping(loaded_slots=loaded)
+
+    assert [e["paint_index"] for e in mapping] == [0, 1, 2, 3]  # positional alignment kept
+    loaded_ams = _loaded_ams_indices(b)
+    bad = [e for e in mapping if e["ams_index"] not in loaded_ams]
+    assert not bad, f"entries reference an empty/non-loaded tray: {bad}"
+
+
+def test_no_entry_points_at_an_empty_tray_with_gap_in_loaded_set():
+    """All occupied slots mapped (0,1,3 loaded, 2 empty). The placeholder at
+    position 2 must not reference the empty tray idx 2."""
+    b = _bridge()
+    loaded = [(0, b._ams_slots[0]), (1, b._ams_slots[1]), (3, b._ams_slots[3])]
+
+    mapping = b._build_auto_ams_box_mapping(loaded_slots=loaded)
+
+    assert [e["paint_index"] for e in mapping] == [0, 1, 2, 3]
+    loaded_ams = _loaded_ams_indices(b)
+    bad = [e for e in mapping if e["ams_index"] not in loaded_ams]
+    assert not bad, f"entries reference an empty/non-loaded tray: {bad}"
+    # Real loaded slots keep their own ams_index.
+    assert mapping[0]["ams_index"] == 0
+    assert mapping[1]["ams_index"] == 1
+    assert mapping[3]["ams_index"] == 3
+
+
+def test_all_full_is_unchanged():
+    """All slots loaded -> no placeholders, identity mapping (the working case)."""
+    slots = [dict(s, status=5) for s in AMS_SLOTS]
+    b = _bridge(slots)
+    loaded = [(i, b._ams_slots[i]) for i in range(4)]
+
+    mapping = b._build_auto_ams_box_mapping(loaded_slots=loaded)
+
+    assert [e["ams_index"] for e in mapping] == [0, 1, 2, 3]