diff --git a/packages/openapi-types/index.ts b/packages/openapi-types/index.ts
index 67d2e6a9..d3a33cad 100644
--- a/packages/openapi-types/index.ts
+++ b/packages/openapi-types/index.ts
@@ -501,7 +501,7 @@ export namespace OpenAPIV2 {
   } & T;
 
   export interface ResponsesObject {
-    [index: string]: Response;
+    [index: string]: Response | undefined;
     default?: Response;
   }
 
diff --git a/packages/openapi-types/tsconfig.json b/packages/openapi-types/tsconfig.json
index 413d47c6..1221d697 100644
--- a/packages/openapi-types/tsconfig.json
+++ b/packages/openapi-types/tsconfig.json
@@ -4,7 +4,8 @@
     "declaration": true,
     "module": "CommonJS",
     "outDir": "dist",
-    "sourceMap": true
+    "sourceMap": true,
+    "strict": true
   },
   "files": [
     "index.ts"