diff --git a/advlabdb/models.py b/advlabdb/models.py
index 4895ff8..b3f7fc7 100644
--- a/advlabdb/models.py
+++ b/advlabdb/models.py
@@ -573,6 +573,16 @@ class Semester(db.Model):
     def lastSemester():
         return Semester.sortedSemestersStartingWithNewest(limit=1).first()
 
+    def num_missing_experiment_marks(self):
+        return db.session.scalar(
+            select(func.count())
+            .select_from(ExperimentMark)
+            .join(GroupExperiment)
+            .join(SemesterExperiment)
+            .where(SemesterExperiment.semester == self)
+            .where(ExperimentMark.final_experiment_mark == None)
+        )
+
     def set_done(self, next_semester=None):
         set_next_semester = next_semester is not None
 
@@ -581,6 +591,13 @@ class Semester(db.Model):
             semester = db.session.get(Semester, id)
 
             if semester == self or not semester.done:
+                num_missing_experiment_marks = self.num_missing_experiment_marks()
+                if num_missing_experiment_marks > 0:
+                    flash(
+                        f"Semester {semester} was set as done, but it has {num_missing_experiment_marks} missing experiment marks!",
+                        "danger",
+                    )
+
                 semester.done = True
 
                 if set_next_semester: