Programmatically Clear Product Group Prices – Invalid Argument for Foreach

I am trying to programatically clear a product's group prices within a custom module. Thisi s my code:

Mage::setIsDeveloperMode(true); // for debug only
try
{
    $product = Mage::getModel('catalog/product')->load($productId);
    $product->setGroupPrice(null);
    $product->save();
}
catch (Exception $ex)
{
    echo "Error: ". $ex->getMessage();
}

When this code executes, I get the following exception:

Warning: Invalid argument supplied for foreach() in
/home/www-data/public_html/app/code/core/Mage/Eav/Model/Entity/Abstract.php
on line 1180

In the system.log file, I see allot of these entries:

2016-03-17T18:01:06+00:00 ERR (3): Warning: Invalid argument supplied
for foreach() in
/home/www-data/public_html/app/code/core/Mage/Eav/Model/Entity/Abstract.php
on line 1180 2016-03-17T18:01:06+00:00 ERR (3): Recoverable Error:
Argument 3 passed to
Mage_Catalog_Model_Resource_Abstract::_canUpdateAttribute() must be of
the type array, null given, called in
/home/www-data/public_html/app/code/core/Mage/Eav/Model/Entity/Abstract.php
on line 1225 and defined in
/home/www-data/public_html/app/code/core/Mage/Catalog/Model/Resource/Abstract.php
on line 543 2016-03-17T18:01:06+00:00 ERR (3): Recoverable Error:
Argument 3 passed to
Mage_Eav_Model_Entity_Abstract::_canUpdateAttribute() must be of the
type array, null given, called in
/home/www-data/public_html/app/code/core/Mage/Catalog/Model/Resource/Abstract.php
on line 545 and defined in
/home/www-data/public_html/app/code/core/Mage/Eav/Model/Entity/Abstract.php
on line 1254 2016-03-17T18:01:06+00:00 ERR (3): Warning:
array_key_exists() expects parameter 2 to be array, null given in
/home/www-data/public_html/app/code/core/Mage/Eav/Model/Entity/Abstract.php
on line 1256

Any idea what might be causing this? The same code on my dev magento setup works, but in production magento, this error started to appear, so I am a little confused.