The $sigmoid expression is the first expression to be implemented as syntactic sugar for nested expressions. See SERVER-92121.

Thus any inputs that are other fields or are expressions themselves will only be resolved at runtime and will be seen first by the inner most $multiply. This means any errors will be handled by $multiply, which might confuse users who do not know about our implementation details. 

We should create a solution so that the error thrown is specific to $sigmoid and not any of it's nested expressions.