The problem in this instance is that the nullsEmpty argument is expecting a boolean. Since many things in python can have a Truthy value, the function assumed that your list of sheetNames was actually you setting nullsEmpty true.
Yes Truthy and Falsy is a thing.
As a general rule of thumb, if you look at a function with multiple optional arguments and you want to skip one in the order, then use keyword arguments (e.g. sheetNames=['name']) to insure you're setting the argument you actually want to set.
Because they are different datatypes, I thought the function detected to datatype to determine the intended use. The flexibility of booleans in python though, have to remember that. Thanks