XForms bind element error - data-binding

Question

I am changing my code to use binds in XForms (which is better practice than using nodesets everywhere!) but I am getting errors.
The error message I receive is: "Error: XForms Error (8): id (data_criterion) does not refer to a bind element..."
From tutorials/guides I have been using, it seems as though this should work, but clearly I am missing something! (btw, I was modeling my binding code after the examples here: http://en.wikibooks.org/wiki/XForms/Bind)
I originally thought the problem was due to the fact I was using xf:select controls as opposed to xf:input like the examples, but even once I dumbed down my code to the most simplistic code, I still receive errors!
This is the model code I am using:
<xf:model id="select_data">
<xf:instance id="criteria_data" xmlns="">
<file>
<criteria>
<criterion></criterion>
</criteria>
</file>
</xf:instance>
<bind id="data_criterion" nodeset="instance('criteria_data')/criteria/criterion"/>
</xf:model>
As for the ui code, this is what I have:
<xf:input bind="data_criterion">
<xf:label>Enter criteria:</xf:label>
</xf:input>
The error message I receive is: "Error: XForms Error (8): id (data_criterion) does not refer to a bind element..."
Anyone have any insight to what the problem is? Also, is there any special usage of bindings and xf:select (with xf:itemset) controls that I should be aware of? (I am ultimately using a lot of xf:select controls on my form..)
Thanks in advance!
EDIT:
I ran the code through this validator, and I got this message (refers to the bind line):
"Warning: Should the following element have the XForms namespace applied?: bind (line 66)"

Answer 1

A couple of things you might want to change:
Not sure of this is the reason for the error, but the nodeset expression should be instance('criteria_data')/criteria/..., without file. Remember: instance() returns the root element, not the document node. (This one you took care by updating the question; good)
You are missing the xf on the bind. It should be: <xf:bind id="data_criterion" nodeset="instance('criteria_data')/criteria/criterion"/>.
See below a full example with your code, which works fine for me under Orbeon Forms:
<xhtml:html xmlns:xhtml="http://www.w3.org/1999/xhtml"
xmlns:xforms="http://www.w3.org/2002/xforms"
xmlns:xf="http://www.w3.org/2002/xforms"
xmlns:xxforms="http://orbeon.org/oxf/xml/xforms"
xmlns:ev="http://www.w3.org/2001/xml-events"
xmlns:xs="http://www.w3.org/2001/XMLSchema"
xmlns:fr="http://orbeon.org/oxf/xml/form-runner">
<xhtml:head>
<xhtml:title>SO Bind</xhtml:title>
<xf:model id="select_data">
<xf:instance id="criteria_data" xmlns="">
<file>
<criteria>
<criterion>Gaga</criterion>
</criteria>
</file>
</xf:instance>
<xf:bind id="data_criterion" nodeset="instance('criteria_data')/criteria/criterion"/>
</xf:model>
</xhtml:head>
<xhtml:body>
<xf:input bind="data_criterion">
<xf:label>Enter criteria:</xf:label>
</xf:input>
</xhtml:body>
</xhtml:html>