diff --git a/django/db/models/sql/query.py b/django/db/models/sql/query.py
index db4e6744bf..dcf89fe8bf 100644
--- a/django/db/models/sql/query.py
+++ b/django/db/models/sql/query.py
@@ -1061,7 +1061,7 @@ class Query(object):
         if (connections[DEFAULT_DB_ALIAS].features.interprets_empty_strings_as_nulls and
                 lookups[-1] == 'exact' and value == ''):
             value = True
-            lookups[-1] = ['isnull']
+            lookups[-1] = 'isnull'
         return value, lookups
 
     def solve_lookup_type(self, lookup):