diff --git a/src/api.ts b/src/api.ts
index 050ed86..d2a7d15 100644
--- a/src/api.ts
+++ b/src/api.ts
@@ -27,7 +27,7 @@ export interface GetToolsOptions {
 }
 
 function isOpenApiDocument(spec: string | OpenAPIV3.Document): spec is OpenAPIV3.Document {
-  return typeof spec === 'object' && 'openapi' in spec;
+  return typeof spec === 'object' && spec !== null && 'openapi' in spec;
 }
 
 /**