[hot-fix] custom field for single

frappe/custom/doctype/custom_field/custom_field.py

@@ -46,10 +46,11 @@ class CustomField(Document):
 			validate_fields_for_doctype(self.dt)
 
 		# update the schema
-		if (self.fieldname not in frappe.db.get_table_columns(self.dt)
-			or getattr(self, "_old_fieldtype", None) != self.fieldtype):
-			from frappe.model.db_schema import updatedb
-			updatedb(self.dt)
+		if not frappe.db.get_value('DocType', self.dt, 'issingle'):
+			if (self.fieldname not in frappe.db.get_table_columns(self.dt)
+				or getattr(self, "_old_fieldtype", None) != self.fieldtype):
+				from frappe.model.db_schema import updatedb
+				updatedb(self.dt)
 
 	def on_trash(self):
 		# delete property setter entries