Using XML node names with hyphens in PHP

I am trying to extract some data from XML but when I execute the following I get a

Warning: Invalid argument supplied for foreach() in ...

Code Example:

foreach ($xml->custom-field-value as $milestone)
{
    ...     
}

It works fine for node names that are single words so I am guessing that it doesn't like the hyphens. Do I need to escape them and if so how?