diff --git a/.changeset/itchy-schools-camp.md b/.changeset/itchy-schools-camp.md
new file mode 100644
index 0000000000..31b4231a69
--- /dev/null
+++ b/.changeset/itchy-schools-camp.md
@@ -0,0 +1,5 @@
+---
+'@backstage/backend-defaults': patch
+---
+
+Explicitly stringify extra logger fields with `JSON.stringify` to prevent `[object Object]` errors.
diff --git a/packages/backend-defaults/src/entrypoints/rootLogger/WinstonLogger.test.ts b/packages/backend-defaults/src/entrypoints/rootLogger/WinstonLogger.test.ts
index ec46f22759..39293dcc55 100644
--- a/packages/backend-defaults/src/entrypoints/rootLogger/WinstonLogger.test.ts
+++ b/packages/backend-defaults/src/entrypoints/rootLogger/WinstonLogger.test.ts
@@ -94,27 +94,20 @@ describe('WinstonLogger', () => {
     );
   });
 
-  it('gracefully handles fields that are not castable to a string', () => {
-    const mockTransport = new Transport({
-      log: jest.fn(),
-      logv: jest.fn(),
-    });
+  it('gracefully handles fields that contain deeper object structures', () => {
+    const log = jest.fn();
+    const mockTransport = new Transport({ log });
 
     const logger = WinstonLogger.create({
       transports: [mockTransport],
     });
 
     logger.error('something went wrong', {
-      field: Object.create(null),
+      field: { foo: { bar: { baz: 'qux' } } },
     });
 
-    expect(mockTransport.log).toHaveBeenCalledWith(
-      expect.objectContaining({
-        [MESSAGE]: expect.stringContaining(
-          '[field value not castable to string]',
-        ),
-      }),
-      expect.any(Function),
+    expect(log.mock.calls[0][0][MESSAGE]).toContain(
+      `={"foo":{"bar":{"baz":"qux"}}}`,
     );
   });
 });
diff --git a/packages/backend-defaults/src/entrypoints/rootLogger/WinstonLogger.ts b/packages/backend-defaults/src/entrypoints/rootLogger/WinstonLogger.ts
index 9f7532e66b..545c9377b2 100644
--- a/packages/backend-defaults/src/entrypoints/rootLogger/WinstonLogger.ts
+++ b/packages/backend-defaults/src/entrypoints/rootLogger/WinstonLogger.ts
@@ -151,7 +151,7 @@ export class WinstonLogger implements RootLoggerService {
             let stringValue = '';
 
             try {
-              stringValue = `${value}`;
+              stringValue = JSON.stringify(value);
             } catch (e) {
               stringValue = '[field value not castable to string]';
             }