[BUG] Count() cannot be the last aggregator in Stats command

[LantaoJin]

What is the bug?

source = $testTable | eval a = 1 | stats sum(a) as sum, avg(a) as avg, count() as cnt by country

Throws

[NESTED_AGGREGATE_FUNCTION] It is not allowed to use an aggregate function in the argument of another aggregate function. Please use the inner aggregate function in a sub-query.;
Project [sum#31L, cnt#33L, country#34]
+- Aggregate [country#38], [sum(a#30) AS sum#31L, count(avg(a#30)) AS cnt#33L, country#38 AS country#34]
+- Project [name#35, age#36, state#37, country#38, year#39, month#40, 1 AS a#30]
+- SubqueryAlias spark_catalog.default.flint_ppl_test
+- Relation spark_catalog.default.flint_ppl_test[name#35,age#36,state#37,country#38,year#39,month#40] csv

But

source = $testTable | eval a = 1 | stats count() as cnt, sum(a) as sum, avg(a) as avg by country

works as expected.