diff --git a/erpnext/schools/doctype/assessment/assessment.py b/erpnext/schools/doctype/assessment/assessment.py
index 003b427..5088336 100644
--- a/erpnext/schools/doctype/assessment/assessment.py
+++ b/erpnext/schools/doctype/assessment/assessment.py
@@ -34,7 +34,7 @@
 			validate_overlap_for(self, "Assessment", "student_group")
 		
 		validate_overlap_for(self, "Assessment", "room")
-		validate_overlap_for(self, "Assessment", "supervisor", self.instructor)
+		validate_overlap_for(self, "Assessment", "supervisor", self.supervisor)
 
 
 def get_assessment_list(doctype, txt, filters, limit_start, limit_page_length=20):