SQL: "Should not fold expression" for conditional function

[astefan]

SELECT IIF(20 = 0, NULL, salary / 20) FROM test_emp

results in

org.elasticsearch.xpack.sql.SqlIllegalArgumentException: Should not fold expression
        at org.elasticsearch.xpack.sql.expression.Expression.fold(Expression.java:72) ~[?:?]
        at org.elasticsearch.xpack.sql.expression.predicate.BinaryPredicate.fold(BinaryPredicate.java:32) ~[?:?]
        at org.elasticsearch.xpack.sql.expression.predicate.conditional.Case.fold(Case.java:132) ~[?:?]
        at org.elasticsearch.xpack.sql.expression.Literal.of(Literal.java:159) ~[?:?]
        at org.elasticsearch.xpack.sql.expression.Literal.of(Literal.java:144) ~[?:?]
        at org.elasticsearch.xpack.sql.optimizer.Optimizer$ConstantFolding.rule(Optimizer.java:1238) ~[?:?]
        at org.elasticsearch.xpack.sql.tree.Node.transformDown(Node.java:174) ~[?:?]

[elasticmachine]

Pinging @elastic/es-search (:Search/SQL)

[matriv]

master : f35c972
7.x : 0c44919
7.5 : 2508cc7
7.4 : 3dc6d58