Prototype cannot read its own Objects?

Question

I need to parse a URL and be sure there are no duplicate keys in the query string. I've converted the query string to an object using String#toQueryParams()

var queryString = this.parseUri(uri).query.toQueryParams();

On an alert, this comes up as [Object object], but...

queryString.each(function(e) {... });

I get the error that queryString.each is not a function. Why? I would assume that prototype made the each() function to make looping over objects easier, so why does this not work? These also return errors:

queryString.isArray();
queryString.isHash();

This baffles me. Why can't prototype read it's own created objects?