[minor] check if joining date is available before validating attandance date
diff --git a/erpnext/hr/doctype/attendance/attendance.py b/erpnext/hr/doctype/attendance/attendance.py
index 72fa47d..458b2dd 100644
--- a/erpnext/hr/doctype/attendance/attendance.py
+++ b/erpnext/hr/doctype/attendance/attendance.py
@@ -35,9 +35,11 @@
 			frappe.throw(_("No leave record found for employee {0} for {1}").format(self.employee, self.attendance_date))
 
 	def validate_attendance_date(self):
+		date_of_joining = frappe.db.get_value("Employee", self.employee, "date_of_joining")
+
 		if getdate(self.attendance_date) > getdate(nowdate()):
 			frappe.throw(_("Attendance can not be marked for future dates"))
-		elif getdate(self.attendance_date) < frappe.db.get_value("Employee", self.employee, "date_of_joining"):
+		elif date_of_joining and getdate(self.attendance_date) < getdate(date_of_joining):
 			frappe.throw(_("Attendance date can not be less than employee's joining date"))
 
 	def validate_employee(self):