diff --git a/src/utils/image.ts b/src/utils/image.ts
index cd6efc9..f778a69 100644
--- a/src/utils/image.ts
+++ b/src/utils/image.ts
@@ -16,14 +16,20 @@ export async function downloadImage(url: string, userAgent: string): Promise<Dow
     maxRedirects: 5,
   });
 
-  const contentType = response.headers['content-type'] || 'image/jpeg';
+  const contentType = normalizeContentType(response.headers['content-type']);
 
   return {
     buffer: Buffer.from(response.data),
-    mimeType: contentType.split(';')[0].trim(),
+    mimeType: contentType,
   };
 }
 
+/** Axios permits non-string header values, but media processing needs a MIME string. */
+export function normalizeContentType(contentType: unknown): string {
+  if (typeof contentType !== 'string') return 'image/jpeg';
+  return contentType.split(';', 1)[0].trim() || 'image/jpeg';
+}
+
 export function bufferToBase64(buffer: Buffer): string {
   return buffer.toString('base64');
 }
diff --git a/tests/image-utils.test.ts b/tests/image-utils.test.ts
index dd13434..0d6db20 100644
--- a/tests/image-utils.test.ts
+++ b/tests/image-utils.test.ts
@@ -1,5 +1,5 @@
 import { describe, it, expect } from 'vitest';
-import { isValidImageMimeType, bufferToBase64 } from '../src/utils/image.js';
+import { isValidImageMimeType, bufferToBase64, normalizeContentType } from '../src/utils/image.js';
 
 describe('isValidImageMimeType', () => {
   it('accepts valid image MIME types', () => {
@@ -72,3 +72,13 @@ describe('bufferToBase64', () => {
     expect(result).toMatch(/^[A-Za-z0-9+/]+=*$/);
   });
 });
+
+describe('normalizeContentType', () => {
+  it('uses the fallback when Axios exposes a non-string header value', () => {
+    expect(normalizeContentType(['image/png'])).toBe('image/jpeg');
+  });
+
+  it('removes header parameters from a string content type', () => {
+    expect(normalizeContentType('image/webp; charset=binary')).toBe('image/webp');
+  });
+});