diff --git a/django/contrib/postgres/aggregates/mixins.py b/django/contrib/postgres/aggregates/mixins.py
index 340a9178793..68f24a5ce3a 100644
--- a/django/contrib/postgres/aggregates/mixins.py
+++ b/django/contrib/postgres/aggregates/mixins.py
@@ -3,18 +3,21 @@ from django.db.models.expressions import OrderByList
 
 class OrderableAggMixin:
     def __init__(self, *expressions, ordering=(), **extra):
-        if isinstance(ordering, (list, tuple)):
+        if not ordering:
+            self.order_by = None
+        elif isinstance(ordering, (list, tuple)):
             self.order_by = OrderByList(*ordering)
         else:
             self.order_by = OrderByList(ordering)
         super().__init__(*expressions, **extra)
 
     def resolve_expression(self, *args, **kwargs):
-        self.order_by = self.order_by.resolve_expression(*args, **kwargs)
+        if self.order_by is not None:
+            self.order_by = self.order_by.resolve_expression(*args, **kwargs)
         return super().resolve_expression(*args, **kwargs)
 
     def get_source_expressions(self):
-        if self.order_by.source_expressions:
+        if self.order_by is not None:
             return super().get_source_expressions() + [self.order_by]
         return super().get_source_expressions()
 
@@ -24,6 +27,9 @@ class OrderableAggMixin:
         return super().set_source_expressions(exprs)
 
     def as_sql(self, compiler, connection):
-        order_by_sql, order_by_params = compiler.compile(self.order_by)
+        if self.order_by is not None:
+            order_by_sql, order_by_params = compiler.compile(self.order_by)
+        else:
+            order_by_sql, order_by_params = "", ()
         sql, sql_params = super().as_sql(compiler, connection, ordering=order_by_sql)
         return sql, (*sql_params, *order_by_params)