Another try

django/db/models/lookups.py

@@ -586,13 +586,21 @@ def as_sql(self, compiler, connection):
         else:
             return "%s IS NOT NULL" % sql, params
 
+    # TODO: This should be nicer
     def as_postgresql(self, compiler, connection):
-        sql, params = self.as_sql(compiler, connection)
-        # TODO: What is the proper check here to be able to cast everything?
+        if not isinstance(self.rhs, bool):
+            raise ValueError(
+                "The QuerySet value for an isnull lookup must be True or False."
+            )
+        placeholder = "%s"
+        # TODO: What is the proper way to figure out the cast type?
         if hasattr(self.lhs.field, "cast_db_type"):
-            cast_type = self.lhs.field.cast_db_type(connection)
-            sql = sql.replace("%s", "%%s::%s" % cast_type)
-        return sql, params
+            placeholder = "(%s)::" + self.lhs.field.cast_db_type(connection)
+        sql, params = compiler.compile(self.lhs)
+        if self.rhs:
+            return (placeholder + " IS NULL") % sql, params
+        else:
+            return (placeholder + " IS NOT NULL") % sql, params
 
 
 @Field.register_lookup