diff --git a/src/index.ts b/src/index.ts
index 4d19d1a2..7e636b6b 100644
--- a/src/index.ts
+++ b/src/index.ts
@@ -77,7 +77,8 @@ export class TypeScriptPlugin {
         exclude: [],
         include: [],
       }
-      fn.package.exclude = _.uniq([...fn.package.exclude, 'node_modules/serverless-plugin-typescript'])
+      // Add plugin to excluded packages or an empty array if exclude is undefined
+      fn.package.exclude = _.uniq([...fn.package.exclude || [], 'node_modules/serverless-plugin-typescript'])
     }
   }