[Fix] Not able to make invoice in the subscription (#14594)

diff --git a/erpnext/accounts/doctype/subscription/subscription.py b/erpnext/accounts/doctype/subscription/subscription.py
index 1edc138..77ceece 100644
--- a/erpnext/accounts/doctype/subscription/subscription.py
+++ b/erpnext/accounts/doctype/subscription/subscription.py
@@ -290,7 +290,7 @@
 		"""
 		Returns the `Customer` linked to the `Subscriber`
 		"""
-		return frappe.get_value('Subscriber', subscriber_name)
+		return frappe.get_value('Subscriber', subscriber_name, 'customer')
 
 	def get_items_from_plans(self, plans, prorate=0):
 		"""