diff --git a/src/services/OpenAPIParser.ts b/src/services/OpenAPIParser.ts
index e5c079e0..26ec31ba 100644
--- a/src/services/OpenAPIParser.ts
+++ b/src/services/OpenAPIParser.ts
@@ -171,7 +171,7 @@ export class OpenAPIParser {
     $ref: string | undefined,
     refsStack: string[],
   ): MergedOpenAPISchema {
-    if (schema['x-circular-ref']) {
+    if (schema?.hasOwnProperty('x-circular-ref')) {
       return schema;
     }