diff --git a/.github/workflows/quality.yaml b/.github/workflows/quality.yaml
index 6b9806ec..15c71577 100644
--- a/.github/workflows/quality.yaml
+++ b/.github/workflows/quality.yaml
@@ -28,17 +28,18 @@ jobs:
           path: .venv
           key: ${{ runner.os }}-quality-py3.12-${{ hashFiles('poetry.lock') }}
 
-      - name: Install Linux test prerequisites
-        run: |
-          sudo apt-get update
-          sudo apt-get install -y xvfb libgl1 libgles2 libmtdev1
-
       - name: Install poetry and dependencies
         uses: ./.github/actions/bootstrap-poetry
         with:
           os: linux
           install-args: --only main,lint,test
 
+      - name: Install Linux prerequisites
+        run: poetry run ./scripts/install_linux_prereqs.sh
+
+      - name: Install Linux test prerequisites
+        run: sudo apt-get install -y xvfb libgl1 libgles2 libmtdev1
+
       - name: Run quality hooks
         run: poetry run pre-commit run --all-files
 
diff --git a/CHANGELOG.md b/CHANGELOG.md
index 603ca7b9..a98fc3d5 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,3 +1,4 @@
+- Fixed: Sanitize missing or malformed spindle values before updating the WHB04 display
 - Enhancement: Add multi-select to the remote file browser
 - Enhancement: Display error message in halt popup. Requires halt errors to start with "ERROR: " in the firmware
 - Enhancement: Add popup notice when using stock firmware instead of the Community Firmware
diff --git a/carveracontroller/addons/pendant/pendant.py b/carveracontroller/addons/pendant/pendant.py
index 51f5c392..92de5f12 100644
--- a/carveracontroller/addons/pendant/pendant.py
+++ b/carveracontroller/addons/pendant/pendant.py
@@ -2,6 +2,7 @@
 
 import json
 import logging
+import math
 from collections.abc import Callable
 
 logger = logging.getLogger(__name__)
@@ -181,7 +182,18 @@ def _handle_display_update(self, daemon: whb04.Daemon) -> None:
             except ValueError:
                 pass
             # end of feedrate fix
-            daemon.set_display_spindle_speed(self._cnc.vars["curspindle"])
+            spindle_value = self._cnc.vars.get("curspindle", 0)
+            try:
+                raw_spindle = float(spindle_value)
+            except OverflowError:
+                # A finite integer can exceed float's range; clamp it by sign.
+                raw_spindle = 65535.0 if spindle_value > 0 else 0.0
+            except (TypeError, ValueError):
+                raw_spindle = 0.0
+            if not math.isfinite(raw_spindle):
+                raw_spindle = 0.0
+            safe_spindle = max(0.0, min(raw_spindle, 65535.0))
+            daemon.set_display_spindle_speed(safe_spindle)
 
             # Update the step indicator to reflect current jog mode
             if self._controller.jog_mode == self._controller.JOG_MODE_CONTINUOUS:
diff --git a/tests/unit/test_pendant_display.py b/tests/unit/test_pendant_display.py
new file mode 100644
index 00000000..11de30e9
--- /dev/null
+++ b/tests/unit/test_pendant_display.py
@@ -0,0 +1,49 @@
+"""Tests for defensive pendant display updates."""
+
+from types import SimpleNamespace
+
+import pytest
+
+from carveracontroller.addons.pendant import pendant as pendant_module
+from carveracontroller.addons.pendant import whb04
+
+
+@pytest.mark.parametrize(
+    ("spindle_vars", "expected_speed"),
+    [
+        ({}, 0),
+        ({"curspindle": None}, 0),
+        ({"curspindle": "invalid"}, 0),
+        ({"curspindle": -1}, 0),
+        ({"curspindle": 70000}, 65535),
+        ({"curspindle": "1234.5"}, 1234),
+        ({"curspindle": float("nan")}, 0),
+        ({"curspindle": float("inf")}, 0),
+        ({"curspindle": float("-inf")}, 0),
+        pytest.param({"curspindle": 10**10000}, 65535, id="huge-positive-integer"),
+        pytest.param({"curspindle": -(10**10000)}, 0, id="huge-negative-integer"),
+    ],
+)
+def test_whb04_display_sanitizes_spindle_speed(spindle_vars, expected_speed):
+    pendant = pendant_module.WHB04.__new__(pendant_module.WHB04)
+    pendant._cnc = SimpleNamespace(
+        vars={
+            "wx": 0.0,
+            "wy": 0.0,
+            "wz": 0.0,
+            "wa": 0.0,
+            "curfeed": 0.0,
+            **spindle_vars,
+        }
+    )
+    pendant._controller = SimpleNamespace(
+        jog_mode=0,
+        JOG_MODE_CONTINUOUS=1,
+        JOG_MODE_STEP=0,
+    )
+    daemon = whb04.Daemon()
+    daemon.set_display_spindle_speed(4321)
+
+    pendant._handle_display_update(daemon)
+
+    assert daemon._display_spindle_speed == expected_speed