Mystery error, 'Class 'Mage_[X]_Helper_Data' not found' (on v1.9.x)

Question

I have a custom module, 'Stl', which I'm using for both a separate model and a SOAP API interface.

In my SOAP API class, for some reason I keep hitting this error message at random with different functions:

<SOAP-ENV:Fault>
    <faultcode>SOAP-ENV:Server</faultcode>
    <faultstring>Class 'Mage_Stl_Helper_Data' not found</faultstring>
</SOAP-ENV:Fault>

This happens for instance when I call functions (in my PHP implementation, in Api.php) like $this->_fault(); or Mage::getModel('mec_stl/product')->save()---but not, say, Mage::getModel('mec_stl/product')->setData().

I've read various people say this Mage_[X]_Helper_Data usually happens if you either don't have your helper class defined, or if you have it improperly defined. The annoying thing is that I have defined the class in my module, and I think correctly, as 'Mec_Stl_Helper_Data':

For app/code/local/Mec/Stl/etc/config.xml:

<?xml version="1.0"?>
<config>
    <!-- ... -->
    <global>
        <helpers>
            <mec_stl>
                <class>Mec_Stl_Helper</class>
            </mec_stl>
        </helpers>
        <!-- ... -->
    </global>
</config>

For /app/code/local/Mec/Stl/Helper/Data.php:

<?php

class Mec_Stl_Helper_Data extends Mage_Core_Helper_Abstract {

}

The blastedly annoying thing with Magento is I have no idea where this is being triggered or how/why.

Any thoughts?

Answer 1

it may come from a mispelling in a xml file. For example, in the system.xml of your module, check if you have some translate="label" module="stl" instead of translate="label" module="mec_stl". Check in the other xml files of your module too.