diff --git a/en/tutorials/book-store/part-7.md b/en/tutorials/book-store/part-7.md
index 7001d81d..17b85974 100644
--- a/en/tutorials/book-store/part-7.md
+++ b/en/tutorials/book-store/part-7.md
@@ -143,13 +143,13 @@ public class EfCoreAuthorRepository
         int skipCount,
         int maxResultCount,
         string sorting,
-        string filter = null)
+        string? filter = null)
     {
         var dbSet = await GetDbSetAsync();
         return await dbSet
             .WhereIf(
                 !filter.IsNullOrWhiteSpace(),
-                author => author.Name.Contains(filter)
+                author => author.Name.Contains(filter!)
              )
             .OrderBy(sorting)
             .Skip(skipCount)