diff --git a/src/lesson.rs b/src/lesson.rs
index fdda432..f65d00a 100644
--- a/src/lesson.rs
+++ b/src/lesson.rs
@@ -1,6 +1,6 @@
 use serde::{Deserialize, Serialize};
 
-use crate::StdClassGroup;
+use crate::{StdClassGroup, StdType};
 
 #[derive(Debug, Clone, Serialize, Deserialize)]
 #[serde(rename_all = "PascalCase")]