diff --git a/advlabdb/models.py b/advlabdb/models.py
index a0e01a4..32533ec 100644
--- a/advlabdb/models.py
+++ b/advlabdb/models.py
@@ -351,6 +351,9 @@ class Semester(db.Model):
     __table_args__ = (db.UniqueConstraint(label, year),)
 
     def customInit(label, year, oldSemester, transferParts, transferAssistants):
+        if label not in ("WS", "SS"):
+            raise DataBaseException("The semester label has to be WS or SS!")
+
         semester = Semester(label=label, year=year)
 
         if transferParts: