diff --git a/erpnext/patches/v11_0/refactor_naming_series.py b/erpnext/patches/v11_0/refactor_naming_series.py
index 3a2cdbd..d5af939 100644
--- a/erpnext/patches/v11_0/refactor_naming_series.py
+++ b/erpnext/patches/v11_0/refactor_naming_series.py
@@ -110,7 +110,7 @@
 	return series_to_set
 
 def get_series_to_preserve(doctype):
-	series_to_preserve = frappe.db.sql_list("""select distinct naming_series from `tab{doctype}`""".format(doctype=doctype))
+	series_to_preserve = frappe.db.sql_list("""select distinct naming_series from `tab{doctype}` where ifnull(naming_series, '') != ''""".format(doctype=doctype))
 	series_to_preserve.sort()
 	return series_to_preserve