diff --git a/lib/snippets/copyright/file_mapper.ts b/lib/snippets/copyright/file_mapper.ts
index 5aa294f..a632820 100644
--- a/lib/snippets/copyright/file_mapper.ts
+++ b/lib/snippets/copyright/file_mapper.ts
@@ -12,7 +12,7 @@ import fs from 'fs-extra';
 export class FileMapper {
   public static async map_all_files (
     folder: string,
-    mutator: Function,
+    mutator: (data: string, file: string, args: Array<unknown>) => string|null,
     args: Array<unknown> = []
   ): Promise<void> {
     const files = await fs.readdir (folder);