diff --git a/src/config.test.ts b/src/config.test.ts
index 5399e80..dc7a409 100644
--- a/src/config.test.ts
+++ b/src/config.test.ts
@@ -40,4 +40,24 @@ describe('loadConfig', () => {
     mockFetch({}, false);
     await expect(loadConfig('config.json')).rejects.toThrow();
   });
+
+  it('ignores malformed renderDefaults fields, keeping defaults and dropping extras', async () => {
+    mockFetch({ streamUrl: 'x', renderDefaults: { lineWidth: 'thick', cutColor: '#abc', bogus: true } });
+    const cfg = await loadConfig('config.json');
+    expect(cfg.renderDefaults.cutColor).toBe('#abc'); // valid string kept
+    expect(cfg.renderDefaults.lineWidth).toBe(DEFAULT_RENDER.lineWidth); // invalid number → default
+    expect(cfg.renderDefaults.rapidColor).toBe(DEFAULT_RENDER.rapidColor); // missing → default
+    expect(cfg.renderDefaults).not.toHaveProperty('bogus'); // extra key dropped
+  });
+
+  it('rejects a calibration whose homography contains NaN', async () => {
+    const calibration = {
+      imagePoints: [[0, 0], [1, 0], [1, 1], [0, 1]],
+      machinePoints: [[0, 0], [1, 0], [1, 1], [0, 1]],
+      homography: [1, 0, 0, 0, 1, 0, 0, 0, NaN],
+    };
+    mockFetch({ streamUrl: 'x', calibration });
+    const cfg = await loadConfig('config.json');
+    expect(cfg.calibration).toBeNull();
+  });
 });
diff --git a/src/config.ts b/src/config.ts
index 7a34a82..d047a8a 100644
--- a/src/config.ts
+++ b/src/config.ts
@@ -1,9 +1,13 @@
-import type { AppConfig, Calibration, Mat3, Vec2 } from './types';
+import type { AppConfig, Calibration, Mat3, Vec2, RenderStyle } from './types';
 
-export const DEFAULT_RENDER = { cutColor: '#00e5ff', rapidColor: '#ff9800', lineWidth: 1.5 };
+export const DEFAULT_RENDER: RenderStyle = { cutColor: '#00e5ff', rapidColor: '#ff9800', lineWidth: 1.5 };
+
+function isFiniteNumber(n: unknown): n is number {
+  return typeof n === 'number' && Number.isFinite(n);
+}
 
 function isVec2Array(v: unknown): v is Vec2[] {
-  return Array.isArray(v) && v.every((p) => Array.isArray(p) && p.length === 2 && p.every((n) => typeof n === 'number'));
+  return Array.isArray(v) && v.every((p) => Array.isArray(p) && p.length === 2 && p.every(isFiniteNumber));
 }
 
 function parseCalibration(c: unknown): Calibration | null {
@@ -11,7 +15,7 @@ function parseCalibration(c: unknown): Calibration | null {
   const obj = c as Record<string, unknown>;
   if (!isVec2Array(obj.imagePoints) || !isVec2Array(obj.machinePoints)) return null;
   if (!Array.isArray(obj.homography) || obj.homography.length !== 9) return null;
-  if (!obj.homography.every((n) => typeof n === 'number')) return null;
+  if (!obj.homography.every(isFiniteNumber)) return null;
   if (obj.imagePoints.length !== obj.machinePoints.length || obj.imagePoints.length < 4) return null;
   return {
     imagePoints: obj.imagePoints,
@@ -20,6 +24,15 @@ function parseCalibration(c: unknown): Calibration | null {
   };
 }
 
+function parseRenderStyle(r: unknown): RenderStyle {
+  const obj = (r && typeof r === 'object' ? r : {}) as Record<string, unknown>;
+  return {
+    cutColor: typeof obj.cutColor === 'string' ? obj.cutColor : DEFAULT_RENDER.cutColor,
+    rapidColor: typeof obj.rapidColor === 'string' ? obj.rapidColor : DEFAULT_RENDER.rapidColor,
+    lineWidth: isFiniteNumber(obj.lineWidth) ? obj.lineWidth : DEFAULT_RENDER.lineWidth,
+  };
+}
+
 export async function loadConfig(url: string): Promise<AppConfig> {
   const res = await fetch(url);
   if (!res.ok) throw new Error(`Failed to load ${url}: ${res.status}`);
@@ -27,6 +40,6 @@ export async function loadConfig(url: string): Promise<AppConfig> {
   return {
     streamUrl: typeof raw.streamUrl === 'string' ? raw.streamUrl : '',
     calibration: parseCalibration(raw.calibration),
-    renderDefaults: { ...DEFAULT_RENDER, ...(raw.renderDefaults as object | undefined) },
+    renderDefaults: parseRenderStyle(raw.renderDefaults),
   };
 }