The document "doc.xml" does not have a valid root (REXML::ParseException)

The document "doc.xml" does not have a valid root (REXML::ParseException)

I'm trying to convert an XML document into a Ruby hash for the first time,
and having no success. I have my XML document, doc.xml, in a folder along
with my script hashrunner.rb.
In hashrunner.rb:
require 'active_support/core_ext/hash'
hash = Hash.from_xml("doc.xml")
puts hash
The first line of the XML document is <?xml version="1.0"
encoding="US-ASCII"?>, if that is helpful.
In my console, when I run ruby hashrunner.rb, I get the error message:
/Users/me/.rvm/gems/ruby-1.9.3-p374/gems/activesupport-4.0.0/lib/active_support/xml_mini/rexml.rb:34:in
`parse':The document "doc.xml" does not have a valid root
(REXML::ParseException)
As someone relatively new to Ruby, I don't understand what this means, and
some internet searching didn't turn up an explanation, either. To start,
I'm not even sure if I'm calling the XML file correctly in the from_xml
method, so please let me know if that's the case. I'd be open to using
different gems or a different approach if that would help.