diff --git a/src/lib/blueprint.ts b/src/lib/blueprint.ts
index f7921635..71e494fb 100644
--- a/src/lib/blueprint.ts
+++ b/src/lib/blueprint.ts
@@ -719,15 +719,24 @@ const createResponse = (
   }
 
   const description = okResponse.description ?? ''
+
+  if (!('x-response-key' in parsedOperation)) {
+    throw new Error(`Missing responseKey for ${path}`)
+  }
+
   const responseKey = parsedOperation['x-response-key']
 
-  if (responseKey == null) {
+  if (responseKey === null) {
     return {
       responseType: 'void',
       description,
     }
   }
 
+  if (responseKey == null) {
+    throw new Error(`Missing responseKey for ${path}`)
+  }
+
   const content = 'content' in okResponse ? okResponse.content : null
   if (typeof content !== 'object' || content === null) {
     return {
diff --git a/src/lib/openapi-schema.ts b/src/lib/openapi-schema.ts
index 564f88fa..0431e558 100644
--- a/src/lib/openapi-schema.ts
+++ b/src/lib/openapi-schema.ts
@@ -60,7 +60,7 @@ export const OpenapiOperationSchema = z.object({
     .optional(),
   responses: ResponseSchema,
   deprecated: z.boolean().default(false),
-  'x-response-key': z.string().nullable().default(null),
+  'x-response-key': z.string().nullable().optional(),
   'x-title': z.string().default(''),
   'x-undocumented': z.string().default(''),
   'x-deprecated': z.string().default(''),
diff --git a/test/fixtures/types/openapi.ts b/test/fixtures/types/openapi.ts
index f6c993fa..d84a2505 100644
--- a/test/fixtures/types/openapi.ts
+++ b/test/fixtures/types/openapi.ts
@@ -246,6 +246,7 @@ export default {
         security: [],
         summary: '/deprecated/undocumented/endpoint',
         tags: ['/deprecated/undocumented'],
+        'x-response-key': null,
         'x-undocumented': 'true',
         'x-title': 'Deprecated and undocumented endpoint',
       },
@@ -272,6 +273,7 @@ export default {
         security: [],
         summary: '/draft/endpoint',
         tags: ['/draft'],
+        'x-response-key': null,
         'x-draft': 'true',
         'x-title': 'Draft endpoint',
       },