[cacti-user] Can't Associate Data Queries to Graph Templates, "Variable passed to each() is not an

Recently I want to custom the Cacti to monitor the processes number of the
server, for example: httpd/mysqld, etc.

So I wrote a XML file, cacti/resource/snmp_queries/processes.xml:
<interface>
    <name>Get Important Processes Number</name>
    <oid_index>.1.3.6.1.4.1.2021.2.1.1</oid_index>

<index_order>prNames:prMin:prMax:prCount:prErrorFlag:prErrMessage</index_order>
    <index_title_format>|chosen_order_field|</index_title_format>

    <fields>
        <prIndex>
            <name>Index</name>
            <method>walk</method>
            <source>value</source>
            <direction>input</direction>
            <oid>.1.3.6.1.4.1.2021.2.1.1</oid>
        </prIndex>
        <prNames>
            <name>Names</name>
            <method>walk</method>
            <source>value</source>
            <direction>input</direction>
            <oid>.1.3.6.1.4.1.2021.2.1.2</oid>
        </prNames>
        <prMin>
            <name>Min</name>
            <method>walk</method>
            <source>value</source>
            <direction>input</source>
            <oid>.1.3.6.1.4.1.2021.2.1.3</oid>
        </prMin>
        <prMax>
            <name>Max</name>
            <method>walk</method>
            <source>value</source>
            <direction>input</direction>
            <oid>.1.3.6.1.4.1.2021.2.1.4</oid>
        </prMax>
        <prCount>
            <name>Count</name>
            <method>walk</method>
            <source>value</source>
            <direction>output</direction>
            <oid>.1.3.6.1.4.1.2021.2.1.5</oid>
        </prCount>
        <!--<prErrorFlag>
            <name>ErrorFlag</name>
            <method>walk</method>
            <source>value</source>
            <direction>output</direction>
            <oid>.1.3.6.1.4.1.2021.2.1.100</oid>
        </prErrorFlag>
        <prErrMessage>
            <name>ErrorMessage</name>
            <method>walk</method>
            <source>value</source>
            <direction>output</direction>
            <oid>.1.3.6.1.4.1.2021.2.1.101</oid>
        </prErrMessage>-->
    </fields>
</interface>

Then I created the Data Queries, Data Input Method is "Get SNMP Data
(Indexed)", and *"**Successfully located XML file"*

But When I Associate the Data Queries to Graph Template, I get this Errors:
Data Template - SNMP - Important Processes
 *Data Source:*  httpd
*Warning*: Variable passed to each() is not an array or object in *
/data/httpd/monitor/cacti/data_queries.php* on line *356*
Now of course I can't create such graph, so what is the problem?

Thanks.

apache-2.2.3
php-5.2.0
mysql-4.1.22
net-snmp-5.1.2
rrdtool-1.2.23
cacti-0.8.6j