diff --git a/gradebook/src/GradeBook/Book.cs b/gradebook/src/GradeBook/Book.cs
index dcf9794..ceafed4 100644
--- a/gradebook/src/GradeBook/Book.cs
+++ b/gradebook/src/GradeBook/Book.cs
@@ -7,6 +7,8 @@ namespace GradeBook
     {
         private List<double> grades;
 
+        readonly string category;
+
         private string name;
         public string Name
         {