diff --git a/server/parser/untis.js b/server/parser/untis.js
index dbba821..7c5f1a3 100644
--- a/server/parser/untis.js
+++ b/server/parser/untis.js
@@ -89,7 +89,7 @@ export function parseSubstitutionPlan(html) {
       });
 
       // Split change if it spans over multiple lessons
-      const rawLesson = rowData.lesson;
+      const rawLesson = rowData.lesson || "0";
       const fromToLessons = rawLesson.match(/\d+/g).map(Number);
       const from = fromToLessons[0];
       const to = fromToLessons[1] || fromToLessons[0];