diff --git a/packages/platform-fastify/adapters/fastify-adapter.ts b/packages/platform-fastify/adapters/fastify-adapter.ts
index 1843b4ba90c..b3476b9da8d 100644
--- a/packages/platform-fastify/adapters/fastify-adapter.ts
+++ b/packages/platform-fastify/adapters/fastify-adapter.ts
@@ -532,9 +532,10 @@ export class FastifyAdapter<
       }
       body = body.getStream();
     }
+    const responseContentType = fastifyReply.getHeader('Content-Type');
     if (
-      fastifyReply.getHeader('Content-Type') !== undefined &&
-      fastifyReply.getHeader('Content-Type') !== 'application/json' &&
+      typeof responseContentType === 'string' &&
+      !responseContentType.startsWith('application/json') &&
       body?.statusCode >= HttpStatus.BAD_REQUEST
     ) {
       Logger.warn(
diff --git a/packages/platform-fastify/test/adapters/fastify-adapter.spec.ts b/packages/platform-fastify/test/adapters/fastify-adapter.spec.ts
index 286e017816d..c4653ac0d22 100644
--- a/packages/platform-fastify/test/adapters/fastify-adapter.spec.ts
+++ b/packages/platform-fastify/test/adapters/fastify-adapter.spec.ts
@@ -54,6 +54,35 @@ describe('FastifyAdapter', () => {
         expect(reply.status).toHaveBeenCalledWith(statusCode);
       }
     });
+
+    it('should keep a JSON content type that carries parameters', () => {
+      const reply = createReply();
+      reply.getHeader.mockReturnValue('application/json; charset=utf-8');
+
+      fastifyAdapter.reply(
+        reply as any,
+        { statusCode: 400, message: 'Oops' },
+        400,
+      );
+
+      expect(reply.header).not.toHaveBeenCalled();
+    });
+
+    it('should force a JSON content type for error bodies sent with a non-JSON content type', () => {
+      const reply = createReply();
+      reply.getHeader.mockReturnValue('text/html');
+
+      fastifyAdapter.reply(
+        reply as any,
+        { statusCode: 400, message: 'Oops' },
+        400,
+      );
+
+      expect(reply.header).toHaveBeenCalledWith(
+        'Content-Type',
+        'application/json',
+      );
+    });
   });
 
   describe('mapException', () => {