diff --git a/django/contrib/postgres/aggregates/mixins.py b/django/contrib/postgres/aggregates/mixins.py
index 68f24a5ce3..527626da4c 100644
--- a/django/contrib/postgres/aggregates/mixins.py
+++ b/django/contrib/postgres/aggregates/mixins.py
@@ -17,13 +17,10 @@ class OrderableAggMixin:
         return super().resolve_expression(*args, **kwargs)
 
     def get_source_expressions(self):
-        if self.order_by is not None:
-            return super().get_source_expressions() + [self.order_by]
-        return super().get_source_expressions()
+        return super().get_source_expressions() + [self.order_by]
 
     def set_source_expressions(self, exprs):
-        if isinstance(exprs[-1], OrderByList):
-            *exprs, self.order_by = exprs
+        *exprs, self.order_by = exprs
         return super().set_source_expressions(exprs)
 
     def as_sql(self, compiler, connection):
diff --git a/django/db/models/aggregates.py b/django/db/models/aggregates.py
index 0cbffacd1b..bf94decab7 100644
--- a/django/db/models/aggregates.py
+++ b/django/db/models/aggregates.py
@@ -50,12 +50,10 @@ class Aggregate(Func):
 
     def get_source_expressions(self):
         source_expressions = super().get_source_expressions()
-        if self.filter:
-            return source_expressions + [self.filter]
-        return source_expressions
+        return source_expressions + [self.filter]
 
     def set_source_expressions(self, exprs):
-        self.filter = self.filter and exprs.pop()
+        *exprs, self.filter = exprs
         return super().set_source_expressions(exprs)
 
     def resolve_expression(
@@ -63,8 +61,10 @@ class Aggregate(Func):
     ):
         # Aggregates are not allowed in UPDATE queries, so ignore for_save
         c = super().resolve_expression(query, allow_joins, reuse, summarize)
-        c.filter = c.filter and c.filter.resolve_expression(
-            query, allow_joins, reuse, summarize
+        c.filter = (
+            c.filter.resolve_expression(query, allow_joins, reuse, summarize)
+            if c.filter
+            else None
         )
         if summarize:
             # Summarized aggregates cannot refer to summarized aggregates.
@@ -104,7 +104,9 @@ class Aggregate(Func):
 
     @property
     def default_alias(self):
-        expressions = self.get_source_expressions()
+        expressions = [
+            expr for expr in self.get_source_expressions() if expr is not None
+        ]
         if len(expressions) == 1 and hasattr(expressions[0], "name"):
             return "%s__%s" % (expressions[0].name, self.name.lower())
         raise TypeError("Complex expressions require an alias")
diff --git a/tests/aggregation/tests.py b/tests/aggregation/tests.py
index 4408535228..075e707102 100644
--- a/tests/aggregation/tests.py
+++ b/tests/aggregation/tests.py
@@ -1291,7 +1291,7 @@ class AggregateTestCase(TestCase):
 
             def as_sql(self, compiler, connection):
                 copy = self.copy()
-                copy.set_source_expressions(copy.get_source_expressions()[0:1])
+                copy.set_source_expressions(copy.get_source_expressions()[0:1] + [None])
                 return super(MyMax, copy).as_sql(compiler, connection)
 
         with self.assertRaisesMessage(TypeError, "Complex aggregates require an alias"):