Extracting variables from parameter query not working

[kevinanderson]

I've copied and modified the sample code for extracting a variable from a query parameter:

<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
<ExtractVariables async="false" continueOnError="true" enabled="true" name="Extract-Variables-Query-Param">
    <DisplayName>Extract Variables - Query Param</DisplayName>
    <Source>request</Source>
    <QueryParam name="appt_number">
        <Pattern ignoreCase="true">{appt_number}</Pattern>
    </QueryParam>
    <VariablePrefix>queryinfo</VariablePrefix>
    <IgnoreUnresolvedVariables>true</IgnoreUnresolvedVariables>
</ExtractVariables>

And this is the URL itself:

http://myproxy.apigee.net/basepath/?appt_number=123

Unfortunately, the data "123" from the query param "appt_number" isn't being picked up. When I try and find the variable queryinfo.appt_number in a trace, it's empty:

- all you see is the variable prefix.

As it's an almost verbatim copy of the sample, it should work - so what's missing?